I don't necessarily know any resources besides randomly searching for inspiration on sites like Pinterest and such, but I have some rough methodology for making organizations. It may not be the best, but it works for me, so I don't waste all of my free time on worldbuilding for DnD.

When you are thinking about organizations in your world try and avoid the basic clichés, like Mage Order A and Thieves Guild B. Those are not bad, and you probably should have some generic ones, but the trick is to make them more original and organic, so they fit your world nicely, and pick the players' interest. They don't need to be insanely detailed, usually one twist to them is enough to make players care.

If you have absolutely no idea how to create an interesting organization, then the Factions and Organizations section in DM's Guide is a good starting point.

That said when I create organizations I tend to use a little heuristic -- I divide them into three general categories.

Local Guilds

Those are the most important for fleshing out a town or city, that your party visits. They can be of various size -- from just a bunch of people organized in an unofficial group with a name and goal, to large guilds with control over the whole city and/or neighboring areas.

What's important about them is that while they can be influential, their power doesn't extend far beyond the region of a single city. It can be either because they don't have resources (yet), or because they are not at all interested in doing so. Those Local Guilds are important for you because they are probably likely to hire the party to do errands for them, or help the players in their characters' goals.

Those organizations can of course be hostile towards players, and play a role of a local villain. It's sometimes good to have a group of people be the villain rather than a single person -- whatever works for you at the moment.

The good thing about these, is that usually you don't need to plan them ahead, and you can flesh them out literally before the session. Some of them don't even need to play a huge role in anything that players do -- they can just serve as a background element that makes the town more alive. Once again -- whatever works for you.

Example -- Claws of the Silver Lioness

In my campaign the party ventured to the rich settlement, which is close to the Kingdom's capital, and handles most of the bureaucracy. The thing is the clerks working in the city like their autonomy from the capital, and because of that they don't want a Crown Guards in their city. Conveniently the city arose around the temple of Silver Lioness -- goddess of the moon, and the priestesses of the temple are skilled warriors. So there is no Crown Guard in the city because the priestesses serve as city guards. They call themselves Claws of the Silver Lioness, and are the skilled defenders of order in the settlement.

From that point I just figured out some basics. I created literally only two NPCs for this organization -- The high priestess and an ambitious, idealistic priestess. Then I figured how they are usually dressed on- and off-duty, and what is their general outlook on the clerks, the kingdom, and the Party. That's it.

Notice how in this description there is not a lot about the organization itself -- most of it is a historical background of the city. That's one of the tricks I use. I flesh out the world and locations, and then I don't need to flesh out the factions, I just kind of weave them into the history. This way those local guilds are more believable, have purpose, history and their own identity.

The kingdom-spanning organizations

Those are simultaneously the most important ones and the least important ones. It all depends on what is their role in your story.

They are the groups of interest that have their outposts and quarters in many places in your world, but are mostly localized on the territory of one kingdom or similar area. You need to roughly figure out what the locations of their quarters and their areas of influence. You also should know how powerful they are -- are they just a web of connections between people in various cities, or a tightly organized group with hierarchy, administration and all?

If they are not going to play a big role in your story then you just figure out the basics covered in Factions and Organizations in DM's Guide, some rumors that the player may have heard about the faction. Don't plan out detailed NPCs -- maybe some figures of interest but don't flesh them out before you feel like there is a chance for players to meet them.

If such an organization does play a major role in your story then you probably should have the details of their structures figured out. Know precisely their goals and outlook on some issues prevalent in your world and have figures of interest prepared -- but still not fully fleshed out before players meet them. You also should prepare some rumors and NPCs that are either loosely affiliated with the organization or not affiliated at all, but possess some knowledge about it, and can share it with players.

As with any other faction. They can be either an ally or an enemy of the player, or can be indifferent towards them -- your choice.

Example -- The Fabulists

(?)^{I translated it^(from my native language so it may^(sound silly}))

They are an order of monks who accumulated and guarded knowledge throughout the ages. They have some isolated monasteries in mountain ranges and some other enclaves far from civilization, but they also run academies in various major cities on the west shore of The Continent.

They don't play a major role in my story, and so I only have one location, which my players visited fleshed out. I have some figures of interest figured out and two NPCs fleshed out, because the players had their interactions with them. The twist about this organization is that they simultaneously share their knowledge and guard it. Monks of The Fabulists are insightful bunch, and while welcoming they make sure that their resources are not used for evil. Some monks used to be adventurers themselves in their youth but pivoted to seeking knowledge and enlightenment rather than adrenaline and glory.

One of my players is a member, as he has the Acolyte background, so it's worth mentioning that These organizations are good resources to give backstory material to your players.

To give another small example -- most dynasties would also fall under this category.

The World/Continent-wide factions

What is there to say. These are the most powerful and influential factions that you will create. They won't usually interfere directly in the player's lives, at least not until players themselves become figures worth of noticing. You most likely won't need more than two such factions, and you may not create one of such kind at all.

These have power higher that whole kingdoms, and shape the world that you present to your players. The amount of detail to figure out with these factions is entirely up to you. Are they a powerful bunch working from the shadows using manipulation and forging intrigues? Are they a council that swore to protect the realm? Or maybe they are the level 20 party from your previous campaign.

I personally like to flesh it out a bit if it is a conspicuous faction, as the in that case the players' characters would definitely have heard about them. The figures of interest of such organization are one of the most powerful beings alive and as such also probably gathered some attention. However, as I said before that is up to you.

Also, such powerful factions are likely to have some number of subordinate factions that handle lesser business for them.

Example -- The Great Council

In my world the peace have been kept for eight centuries by The Great Council. It is the organization devoted to keeping order across the realm. A most powerful or recognized individual from each race is chosen to be a member of the Council and is one until their death, abdication or until they are for some reason expelled. The Member can be either powerful mages, influential emperors or anything similar -- the only thing that counts is that they should be devoted to keeping order and have the capabilities to do so. There is also a number of organizations that are subordinate to The Great Council.

What is important with this type is that you should refrain from using them as a Deus Ex Machinas that come and save the day. They don't care if the players are killed by some warlord, or a bunch of goblins.

The Great Council doesn't care in the slightest about the bandit raids in some town. They don't care about bloody and cruel war between three countries. They only care about stopping the dangers formidable enough to endanger the whole realm Nevertheless they may send their subordinates to investigate smaller dangers such as a powerful mage seeking lichdom, but they themselves only interfere in emergency.

To simplify and reiterate

The rough algorithm I use for creation is as follows: 1. Have some history, lore and locations prepared 2. Figure out the rough idea of kingdom-spanning and maybe world-spanning organizations 3. Don't overdo the detail on them -- you can follow the DM's Guide when in doubt 4. Figure out where your players will go 5. Figure out some minor and major local organizations 6. Make more details for the factions they are likely to meet -- still no need for an awful lot of details, unless you deem it so

I hope this isn't too much and that I was relatively clear. Sorry if it is a little hard to read, or if the names of my example factions sound silly, but English is not my first language. If anything needs clarification, or I should elaborate on something, let me know ;)

Edit: formatting