Can you make a tour of the bridal/prom shops in your area? Since it is well before "prom season" you might be able to snap up some bargains; if the prices are right, you might be able to get your dress for this year and for next.
Or if you went to proms in highschool, you could recycle your old prom dress, if it still fits and you like it all right.
Otherwise, I would suggest having a confab with a local seamstress and seeing if there is time to have a dress custom made. I know I could make something like what you are after for under $600, possibly under $300, considering the biggest, froofiest wedding dress I did was just under $500 including materials. I think my charges are in line with those of most dressmakers, but it would be worth your while to make independent inquiries.