You don't say what level you are dancing. If it's bronze, you could probably buy a regular dress-something sexy with a skirt that moves--and perhaps add some stones yourself.
A good source for inexpensive ballroom and latin dresses is rainbowshiu in Hong Kong. I don't remember the website address, but if you google her, you'll find it. She also sells on ebay. She is very reliable.
Most of her dresses use Korean rhinestones, which I think are not worth using. They really don't show up on the floor at all. The best, of course, are Swarovski, but Czech Preciosa stones are also good and cheaper.
My recommendation would be to buy an unstoned dress from her and stone it yourself. Don't worry if you're not "crafty." I'm terrible at those sorts of things, but I can stone. But practice on a piece of scrap cloth before you start on your dress.
The easiest patterns are stones scattered randomly over the dress. Or get one with lace trim and stone the lace, following its pattern. A good resource for rhinestones is rhinestoneguy.com. His website also has instructions on how to stone.
Good luck! And have fun at the comp!
