Preownedweddingdresses.com, Still White, and Tradesy like others mentioned are great. Also, check Facebook Marketplace & Poshmark! I looked at all these sites but ended up getting my dress from FB Marketplace! It was brand new without alterations. The other bride just decided not to wear it.
I got my dress off StillWhite and am so happy I did. I paid 1/6th of the price for it new, and it already came with a bustle added so the additional alterations I will be paying for are minimal (I could actually wear it w/o any alterations, but I lost a little weight and want it to be more fitted). Total cost is well under budget of what I wanted to spend, and less expensive and much nicer than anything I tried on in the bridal shops.
My one tip with StillWhite is to limit searches to USA or North America only. There are a lot of dresses from sellers in the UK and Australia and shipping overseas is expensive plus if there is an issue I think it will be more challenging to dispute.
Girrrrlll, listen! You can get a Nrand New custom fit dress for under $400 - $500 depending on how elaborate you want it to be. My daughter tried on a dress a a well known bridal salon and it was $1500. We found a Very Similar dress with more ruffles and a longer train, w/ bling, and she also ordered a second reception dress for around $400. That's her story. Now mine. I'm now engaged and went to the same local salon. The dress I fell in love with was $900 on clearance. I found a similar style less elaborate for $500. I decided to go to the site she ordered her dress from and found a dress that was a PERFECT combo of the two dresses. I cried on the spot because I knew it was my dress! The company is DHGate. It is an overseas, online store but I can promise you the quality is Amazing and the price is even better. You must have a seamstress (who knows how to take all measurements properly to measure you. Then at checkout, you'll enter all of those numbers. Your dress will arrive custom made for you. Another company is Jjs House! Thet also sell other accessories like veils and petticoats for Way less than a bridal salon! Happy shopping and Best Wishes!