Most Helpful Girl
I would go with Rent the Runway. They offer real designer gowns for cheap rental prices (plus you are probably never going to wear a dress again, so renting is awesome).
Try JJHouse. com for a wide variety of dresses from poofy to sleek at reasonable prices.
https://www.modcloth.com/ if you want something a little vintage or a little more on the quirky side.
You can try Asos for some lovely dress options with bonus that on the site, you get to see them walked on a runway if you click the video on the side of each photo.