buychina
Amazon
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.
I'd say pigtails.
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
You can buy them anywhere online; remember (especially when it comes to hair) you get what you pay for. If you buy a wig because it's cheap, you'll get cheap hair.
Try ebay.com, they have a large variety of high quality cosplay wigs.
Hello Cosplay
Most cosplay wigs are meant to act as regular hair..so...yes.
Hello Cosplay or Amazon
Cosplay is never entirely cheap, unfortunately. But, I have seen many available on both Amazon and Ebay.
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.
I bought twice from link below. A trusted wholesaler I think but not a fast shipper.