I don't know if you're looking for professional clothes, if that's the case then online is the best place for that. But i highly recommend buying from Consignment stores! You know, Goodwill, that kind of stuff. This is where i put together all of my costumes from, unless its really complicated such as Chi from Chobits and her dress. But if its a school girl uniform type (such as Amu from Shugo Chara) they ALWAYS have school skirts and things like that. Be creative! And if you're entering a contest, they like the things YOU create much better than just something you bought online. :)
Try checking a cosplay site such as Cosplay Magic or Cosplay House. There are also people you can hire on consignment to custom-made costumes for you.
in ebay.com
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.
Buy them online. Your best bets are eBay and lightinthebox. I got a Shikamaru Nara costume from LITB.
Most cosplay stores sell mortal kombat cosplay.
Well if it's just for cosplay you can buy it on a cosplay website (full costume)
i would suggest checking amazon or eBay, and if you don't find it, google/Bing cosplay supply websites. most cosplay you can't buy at stores unless you have a local cosplay shop.
Buy the costume.
Wear clothes like Riku.
Buy a cosplay or ask someone to make it for you. Most cosplay websites will make custom stuff. So do some searching.
try Ebay, there is alot of cosplay items available there
you can get it from cosplay.com