tape to keep your clothing from moving:
Amazon.com: Braza Flash Tape (1009): Clothing
I recommend checking out the second hand stores. Look in all size ranges. If you can sew, you may be able to alter a large size gown into something special. I've found dresses and sweaters with great appliques or sequined areas that can be re-used.
In general, I actually seem to find great clothing when I'm browsing and not shopping for a specific event. Especially at end of season sales.
A long skirt, dressy blouse, and nice jewellery might work. DSC_1654 | Flickr - Photo Sharing!