Can't recommend an online wedding dress website for that budget (sorry!) but I can tell you to beware of MOST of the ones that claim to offer grand dresses for tiny prices. Most of them are knockoff sites and take designer dresses and "recreate" them (poorly). For every one "accurate" recreation, there will be 100 epic fails. So if it looks too good to be true, it probably is!
Your best bet if you want a brand new dress for that low of a budget will probably be to hit somewhere like David's Bridal during their $99 sale, find bridal salons having sample sales or consider getting a dress that's not really a "wedding" dress - think a prom dress in white, nice dress from Nordstrom's in white, etc.
If you don't mind a pre-owned dress, look at some of the bridal resale websites. There are tons of options out there if you're willing to do the digging!