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
Current Weight(08/25/14) - 287.5
Short term goal (08/31/14 - or sooner!) - 285.0
Long term goal (02/28/15) - 199.0 (ONEderland!!)