They are actually pretty tough to find. Your best bet is to have some custom made for you.
It's possible to cosplay anything. As for being able to find the cosplay at a store or something that would depend on the store...but it would be easiest to make one...
The Foodnetwork Store
Go to an online store, find the costume, buy it, wear it. ^_^
The store in the link below does custom cosplay work. you can try online cospay store your-cosplay.com
Do a search on a cosplay store or ask someone to make it custom.
at amazon or a cosplay store
Suggestions:EbayAnime cosplay sitesYour local anime storeYou can make it yourselfOr find a store online.
You can find a Alphonse Elric cosplay at the related link below.
Sadly, and despite the mounting popularity of cosplaying, finding a cosplay "store" is an impossible task. I've only ever seen 2 stores and both very sad. I recommend just looking online. There are a ton of good costumes out there. Look for an American store like cosplay locator.
From Kingdom Hearts? Go to the cosplay store in the links. Do a search for the costume or ask them to make it custom.
Wait, actually there is a place called the magic wardrobe you might want to chaeck out.
If you need a cosplay fast and cheap the best place to always go is the thrift store that's where I have gotten all of my cosplay. (And I cosplay S. Italy)