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.
If u need to get the DVD for it then try checking out the Media stores.....well i think they have not arrived yet!!
Do you have long or short hair? Well i guess you can buy a wig ^_^ L from Death Note's pretty easy, just find a plain white jumper, black trousers and some eyeliner for the eyes, depending on the length of your hair it might be good to buy a wig aswell.
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)
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