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Death Note -- L. Most people have L like clothes, and if your hair isn't the right color/style it shouldn't be too hard to get a wig.

Kuroshitsuji -- If you have any kind of fancy suit, you can go as one of the butlers.

Avatar -- A long green skirt, long-sleeved green shirt, and armor/accessories made out of cardboard or something for a kyoshi warrior.

Anything with a simple school uniform should be easy enough. You just need a skirt and a vest/blazer, or you can modify a t-shirt into a seifuku (you can find tutorials for this online).

If you can sew or find similar clothes, a Sasuke costume shouldn't take too long to make (I dunno what you could do for sandals if you don't already have them, though)...