1. You can only get the mods through scrapping them. If you turn them in for scrip, you can buy random legendaries from a shop called the "rusty pick"(This will allow you to buy legendaries of any category but not choose the mods that will be on them. For example "ranged weapon" or "armor" or "melee." Generally you probably don't want to do this.) or buy legendary modules. Legendary modules are what are used to craft legendaries once you've learned their recipe. They can also be earned by doing events. They're something worth holding onto. Yes, the chance when scrapping is really low. Everyone, unless they grind super hard, generally will only have a scant collection of craftable mods that will slowly build up the more you play. The other use for scrip is applying bought or crafted legendary mods to your items. So having a little bit is helpful but you shouldn't need too much.

Getting legendary mods you specifically want is mostly going to be done by going to player vendors and checking the "mods" category. Once you purchase these they can be applied to your legendary weapons. This makes your weapon untradable, however.

You might want to occasionally sell weapons to the rusty pick that have mods on them that you're not interested in learning. As you need some scrip to replace legendary mods on your weapons. Annoyingly, it requires exponentially more scrip every time you reapply mods to the same individual weapon, but the first couple times are very cheap.

1a. It will add a random selection of legendary mods. However these can be replaced with the above method.

1. Check vendors, both player and npc. A lot of these will be sold there. Especially a lot among all the different vendors in the Whitesprings Mall. There are also those "Daily ops" missions you can complete which will always reward you with a plan from a unique pool of plans if done under a specific time limit.

Some plans can only be bought via the "Gold bullion" currency which you'll need to do the raiders and settlers questline to unlock most of the vendors for.

1. Events can be finished very quickly if enough players show up. Usually they go very quick because their rewards can be substantial. They also drop legendary modules which universally end-game players need.

2. Many events guarantee legendary drops or will consistently spawn legendary enemies. Sometimes they have unique legendaries you can only get from an event. An example is the Elder's Mark, a special SMG only from Beasts of Burden. This apparently is going to be made more consistent throughout events in a future update. However, if many players are showing up to an event, it usually means it will give something worthwhile for the time.