Local consignment stores are usually FULL Of dresses this time of year. I have also been seeing many on the local FB yardsale groups. Most of the time they list when/where it was worn so that you can avoid wearing it in the same school district if you are concerned. Sometimes the sellers also have shoes and/or bags to match!
Just understand when you buy used (and probably cannot return) what CAN and CANNOT be done as far as alterations are concerned. Beading and boning makes it difficult and expensive (but not impossible) to take in the bodice. Some fabrics lend themselves to hemming the bottom better than others. The easiest alterations are things like taking up spaghetti straps or sewing in a bra.
Good luck!...............P
Actually, you can't return new prom dresses, either. Dd19's last one was one she found online. She went to a store that carried that designer, but they didn't have it in stock. She tried in a similar one to get the sizing, and hers did end up fitting, but if it didn't, it was still ours.
A less expensive option is Nordstrom. They have a lot of dresses online, free shipping to and from (very fast), and you can return. I shop here for dresses for dd14 (15 in a few weeks), because she is very petite, and everything has to be altered. I can bring dresses to the tailor, and they tell me if it can be done or not.
She better get moving though - brick and mortar stores are going to have a small amount left, which will have been tried on a million times already. Ds17' girlfriend has had her dress for a couple of months, already posted to the prom dress Instagram group (so there are no duplicates).
ETA, beware of buying online. Legit sites will be more expensive, but no returns, and the ones with much less expensive dresses are more likely scammers, putting the picture of a legit dress on their website, but sending a poorly made copy.