Deb always has some rather nice dresses but I don't know if they're too dressy for the occasion. But if the theme is Hollywood, I would assume that most people will be pretty dressy. I wouldn't recommend going to the usual mall stores (Kohl's, Sears, JCP, etc.) because at this time of year, they usually have flowery sundresses because spring is coming up,or the same dresses that they always have. You won't find anything unique.
I don't know where you'd be able to get an inexpensive dress. Most dressy ones on the market are at least $50.