I’ve gotten nice dresses from Jewish stores, department stores online and department stores in person. If you live near a Lord and Taylor, Macy’s, Century 21, Bloomingdales, Nordstrom, or any area equivalent, they usually have nice stuff. (Some of the above are mucho expensive, but if you’ll use it enough and don’t get something too fancy, so that you can use it for Shabbos and Yom Tov, you may find it’s worth it.) Online are the websites for the above stores and also Zappos, Amazon, etc.
For my last two cousins’ weddings I got dresses from Nordstrom online (discount) and, wait for it, David’s Bridal. If you’re wearing long, that might actually be a great resource- they have nice stuff and are really not THAT expensive (my sister got hers for $80).
Of course, for most of this you’ll need a shell and/or skirt underneath. If you don’t like that idea, then I have no idea what you should do- sorry… 🙂 I’ve never shopped in Brooklyn, so I have no idea if they have any already-tznius dresses there, but if you’re gonna be in Brooklyn and it’s that important to you then it may be worth getting there and looking.
Good luck!