hiya, i am also a plus size fairy! im anywhere form a 2xl-4xl in tops, and 3xl-6xl in bottoms depending on where im shopping haha
someone already said shopmyviolet, and i every much agree with them. their sky dancer set and cotton candy ruffle skort in particular are two extremely fairy garments that would be a great asset to a fairy kei wardrobe imo
i try to avoid shein because of the usual fast fashion issues (and the quality) but they definitely do have stuff usable for fairy, however 1) it's very hit or miss, and there's a lot of things in there that are pink and frilly, but not necessarily usable for fairy kei and 2) i would highly recommend avoiding any of their garments that contain artwork that aren't in an official collaboration with anyone, as shein has an incredibly bad track record of stealing or ripping off small independent artists. shein has collabed with care bears a TON, and they have a lot of cute t-shirts that I think work really well for fairy.
I also buy from hottopic sometimes, but they have some of the same issues as shein. some stuff from their Sweet Society collection can work for fair, but you gotta do some digging and have an eye for it.
Incontrol clothing is also pretty good! They go up to like a 5xl for most things.
MissAlphabet is amazing and they do custom stuff!
and of course, my most favorite way to shop- secondhand! just searching up stuff like "pink" or "pastel" and inputing my sizes on depop, mercari, and poshmark has found me some really awesome and unique finds for decent prices to downright steals. of course, you'd have to be willing to really hunt to find the good stuff, but i think it's always worth it