I bought my daughter's prom dress off Ebay, 2 years ago. She is a bigger girl and the selection locally was abysmal. Funny thing though, she settled on the 1 dress we did have an option to purchase locally (though they would have had to order in the colour and size). I saved over 300.00 (just about 1/2) by ordering online.
Be careful... read the fine print, and I would recommend staying away from anything marked irregular or seconds. Aside from that you should be fine.
Search for a style you like, see if can find the designer, then go directly to their website
Good luck and happy shopping!
And if this is for you
Start (01/10/14) - 352.9
FitDay Start (02/28/14) - 333.7
Restart Weight(08/10/15) - 312.0
Current Weight(08/14/15) - 304.7
Long term goal (12/25/15) - 270.0