I don't know anything about your style or budget, but I always find great dresses at Lord and Taylor. They have stores in most major cities and here is their website: link
Try going for a great classic cocktail dress in a fun color that you can wear later on and all of you friends will be jealous when they have to deal with huge glittery gowns and you're free to dance and have a great time!
Before prom, me and my friends would go to different websites and look at a bunch of dresses just to get an idea of what we wanted ( link a lot of girls at my school looked at this site). At one point, we went to the mall and tried on dresses for fun. But I got mine from a boutique with handmade dresses. It wasn't cheap, but it was a really nice dress. If you know of any places like that in your area, you should see if they have a website.
One of my friends ended up getting hers custom made from a dress shop in the area. If you have an idea of what you want but can't find it anywhere, that's an option.
Depends on the style you want and what suits you if you want a cheaper prom dress try lipsy I had a beautiful red with red lace and sparkly bits (floor length) from their range and it was cheep too ( in the sale) lots of girls had lipsy dresses at my prom. The many different styles cover a range of body types and can be very flattering. I now buy them for social functions and the presidents ball. (you could not even tell that I was 7months pregnant when I wore my dress for the presidents ball at uni.
I adore dresses! A lot of people at my high school bought their dresses at BCBG, but if you go to local boutiques they have really nice stuff and you can bet it'll be unique. Although they may be a bit expensive they'll usually be much cheaper than many other stores.