Where should i go to buy a prom dress?

my high school prom is coming up! i have the date, but i wanna look absolutely stunning :) i'm even going to try to watch what i eat so i can feel more confident when i wear this dress :) (i'm not super fat, but can stand to drop a few pounds :P) i'm looking for a white floor-length dress that poofs out. (like a ball-room dress) but one that isn't too expensive i'm looking to pay less than two hundred.