I'm not sure of anything specific for dancewear, but you should be able to find some good support undergarments at a local lingerie or smaller department store--you know, the kind with the old ladies that actually measure you to get the bra to fit right! I have an "UnderWonder" brand all in one that works well. Of course it depends on the dress you're wearing, some of them have built in support.

One thing--don't have so much support that you cut off your circulation. You'll likely sit a bit at your showcase, and I've found that the "Almost Bare" hose by No Nonsense are the only ones that keep me comfortable after a day of dancing, sitting, dancing, sitting. Good luck!