Many cosplay sites take "commissions" which means they will make you anything you want. Your costume will look amazing, but it won't be cheap!
you can try online cospay store your-cosplay.com
Try doing something different and exciting - something that will make her party special. It would be fun and memorable to have a magician or cosplay princess there. There are plenty of children's magicians and cosplay princess' here: bit.ly/1ICYL3j
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.
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.
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
There should be a cosplay store in the United States, but if worst comes to worst you can just buy cosplay online.
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)
Google Directory Cosplay http://www.google.com/Top/Arts/Animation/Anime/Fandom/Cosplay/ Anime Costume resources: http://anime.about.com/od/cosplay/tp/aa12.htm