Where can I find a teen's Hermione Granger costume?

I'm going to see HPatDH with my friend (as every Potter lover will) and she's absolutely set on dressing up as the characters. I want to go as Hermione Granger but I can't find a single half-decent outfit. I'm a size 10 and I don't want anything revealing (it is Hermione the nerd anyway!) Thank you!