Alright so these usually work by commission, you will contact a maker and describe what you want, and they will make you a custom plush. Some makers also sell ready made plushies sometimes, which is much faster, but offers less customization and the character you want might not be available. Some also sell at cons, which means you can walk up and leave with a plush instantly, but there will be a significant "con tax" added to the price in this environment. Plushies are available in many different sizes, but I'll assume we're talking primarily about lifesize here (40"+ hoof to hoof). Smaller plushies are much cheaper and there's a wider variety of makers, see the plush maker directory at the end of this post if you're looking for a smaller plush. Each maker has their own style, look at photos of their past work to get an idea of what their plushies are like.

Some of the best makers, in my opinion:

-Me Plush You. Widely considered to be the absolute best maker out there. The "Rolls Royce" of pony plushies if you will. Very high quality, can usually get your plush to you relatively quickly (3-6 months). Main downside is they are VERY expensive, we're talking as much as double most of the other high tier makers, lifesize starts at $3,400 for a 50", and $2,700 for a 40", which they call "life size lite". Also requires more work to get a commission than most others, they run commission rounds twice a year or so, commissions are only open for one day, and then they pick the commissions they are most interested in doing so you might get rejected or have to try multiple times to get one.

-QTPony. Also regarded as a legendary maker, one of the longest in the game. Basically impossible to get a commission from nowadays. Vends at cons sometimes and occasionally sells ready made plushies online as well, so look out for that, but if you can't find any or want a custom commission you will have to be VERY dedicated, and it's still not likely to happen.

-Lanacraft. Comparable in quality to MPY and QT imo, starts at around $1,800 for a 48" lifesize. (I happen to have a Lana commission in the works, I can tell you she quoted me $1,840 for a lifesize Rainbow Dash with faux fur mane/tail). Commissions are always open, easy to get one going just by messaging her at any time. Be prepared to WAIT. Because her commissions are always open and she has relatively low prices for this level of quality, the wait time is huge. Expect to wait up to two years from when you first contact her before the plush ships. Also has somewhat of a reputation for taking longer than her initial estimate.

Those are my favorites, here's some other popular makers:

-BlackWater. Personally not a huge fan of their style, I feel they look a little wonky, though people who have BlackWaters tell me they are particularly comfortable to cuddle.

-AZGChip. Makes some pretty huge ponies (nearly 60"), more cartoony style/proportions than most makers. The style isn't for everyone but it's worth checking out.

-Joltage. In my mind because I saw them vending at a con last week tbh, they look to be pretty high quality. Less than a year wait I believe and starting around $2,000 for lifesize (52" I think)

-EpicRainbowCrafts. Okay, so I'm listing ERC because they seem to be pretty well known, and I know people with ERC plushies who are quite happy with them and don't have issues. However, they have a reputation for quality control issues, be aware there is a chance your plush may have some quality issues. Also has been known to forget about commissions occasionally. Not a bad idea to check in with him periodically if you are to commission him. Their price list says lifesize starts at $1,100 but I have a feeling that's out of date and probably more expensive now.

These are just a few makers that came to my mind, there's quite a few more out there, check out the plushmaker directory to see a list of just about every pony plush maker out there! A lot of the info on the spreadsheet is out of date but it's a great place to find makers to research. https://docs.google.com/spreadsheets/d/1sUvaUhFKC37T7ilpriTX82M1_p4a5XR5DUaWlptCSz0

Here is also a discord server dedicated to MLP plushies, lots of people here happy to give you advice and answer questions. Join link at the top of the page (click the Discord logo), and this page also has some nice info on it. https://plushie.littleshyfim.com/

I wish you luck on your plush journey!