Help Me!!!?

Prom is comming up May 5th and I don't have a dress! I need to know where to get a dress and maybe what color/style to get. I will not go any shorter than floor-length. The theme is "An Unforgettable Evening" and the colors are burgundy, midnight blue, and silver. Also, since my mom is buying the dress for me, cost is a factor. I cannot go above $150. Where can I go to look for cheap but elegant prom dresses?!