Your best bet is probably to look for vintage, indie, and offbrand, unless you've got a ton of money to spend. If you look on eBay, you can nearly always find cheap t-shirts and accessories-- I've always had good luck finding things like vintage Care Bears or My Little Pony shirts very cheaply. If you're crafty, searching for cute vintage fabric/bedsheets to cut up and make dresses or skirts from is always good!
Bodyline, while it mostly sells Lolita and Punk clothing, has some cute petticoats and bloomers and even a couple of skirts/salopettes that would work well in a fairy-kei coordinate. They also sell really cute shoes! The pricing is pretty cheap and they ship internationally with a flat rate $10 postage fee.
Refuse To Be Usual is a bit more expensive, but compared to the brands, it's still cheap. Their 'cute-me-up' section has quite a lot of really sweet shirts and jackets.
Mystere Wonderland is a little less expensive than RTBU, and it sometimes stocks brand items, too. Like RTBU they sell a lot of GLP/MarJour clothes, but it's all pretty nice. They also sell the odd 6% or Milklim replica.
EGL Comm Sales is always worth a look. It's mainly Lolita, but allows any items sold from generally-Lolita brands (Angelic Pretty sells a lot of fairy-kei appropriate clothing), and sometimes you can find real bargains on there.
80s Cuties is pop-kei@LJ's sales comm. There isn't always much to look at, but sometimes you can find good second-hand brand clothing there.
VK Sales is mostly visual-kei and punk, but it allows any J-fashion style, and you can find people selling fairy-kei appropriate items there.
Storenvy,
Etsy,
Artfire, and
BigCartel are all places to look. Look for 'kawaii', 'lolita', 'fairy kei', 'sweets jewellery', etc.
Chocomint Rakuten ships internationally, and sells gorgeous accessories.
Cute Can Kill is US-based and sells adorable limited jewellery. And of course don't forget thrift stores, charity shops, and flea markets. :) You often find kitschy 80s sweaters or cardigans that are absolutely perfect for bulking up your wardrobe. If you're small, I've often found really cute clothes in the children's section. Plus, it's always cheaper than adult clothes, haha. Plastic children's jewellery is also perfect for gaining some accessory bulk-- maybe not statement pieces, but definitely nice casual things that you don't mind getting wrecked if you're wearing fairy-kei on a daily basis.
If you wear them,
Mintymix sells a variety of wigs (plus some cute jewellery).
Gothic Lolita Wigs are also really nice.
Alice Fururun is a second-hand Japanese brand store, and it's mostly Lolita, but you find some really nice casual Angelic Pretty, MILK, and Emily Temple Cute items there on occasion. You can of course buy directly from places like the Milklim, 6% Dokidoki, Angelic Pretty, and Spank! webshops, but you need a shopping service for all of them except AP, and, well-- it's pretty damn expensive. If you've got more money than you know what to do with, though, that's fine. xD
The best thing to do is really to look around, check eBay regularly, keep your eyes open for bargains, and don't be afraid to modify or even make your own accessories and clothes. Plus, be inventive: a hideous skirt with a cute print could make a sweet strapless dress, worn over a print t-shirt and some bloomer shorts, and cinched in with a corset or wide belt. That gross denim jacket could become an amazing vest, with the sleeves cut off and some lace, bows, cute badges, and iron-on transfers added to it.
Hope this helped a little, and good luck!