Well I don't know how fancy a dress you're planning on going for, so my suggestions may vary but lets see...
occasionally wears this white dress, though its more on the casual side and probably not best suited for a graduation event. Still, its cute.
I don't know this series very well, but I remembered this character from Fullmoon wo Sagashite wore a white dress, picture here:
from Gankutsuou: The Count of Monte Cristo has a beautiful signature gown that she wears at the opera, this one would be nice. I can't find a great picture or even a decent screencap at the moment, but there are a couple cosplayers here who have made the dress if you'd rather look that up.
Anyways, that's all I got for now. Good luck!