Try cosplaymagic.com ~ see the related link below .
I don't believe that there are much stores that sell Final Fantasy 10 cosplay costumes, so your best bet would to be checking online. Online stores have great selections of Final Fantasy cosplay costumes.
There are tons and tons of different cosplay shops online. I've linked to them below.
Go to an online store, find the costume, buy it, wear it. ^_^
I know cosplay locator makes crazy custom stuff like that. But be prepared to pay out the wazoo.
Ebay
Most cosplay stores sell mortal kombat cosplay.
This is all I can find: http://www.cosplaymagic.com/nashabcowig.html
you can buy a cosplay costume online or you can make a costume yourself. There are also many places where you and other people can cosplay known as cosplay events.
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.
you can always buy online, however i find shopping in consignment much better. you can try online cospay store your-cosplay.com
Your local michaels or Joannes are good bets.
You can find a Alphonse Elric cosplay at the related link below.