It's most economical to make the costumes yourself - check out http://forums.cosplay.com for advice and help.

If you still want to pay for a costume, check out "cosplay final fantasy" on eBay, or consider commissioning the costume from another cosplayer. Some reputable commissioners are:

http://www.dream-angels.net/
http://limebarb.com/cosplay/front.htm
http://www.erikadoor.com/cosplay/main.html