Most competitions give rules and guidelines for attire in the rules and regulations. Check those out to see what is allowed for the comp before ordering anything.

If you want to make a dress, I've seen patterns on ebay. If you prefer to buy and don't want to spend a ton of money on a dress that she will soon outgrow, check out this site: www.fotop.net/rainbowrainbow
They will make anything you want. You just need to send the measurements and a picture or sketch of the dress you would like. You can also browse the site and have a dress on the site recreated. Rainbow is the lady's name. Her prices are very low. The dresses come from China.
I have another contact in China. Her name is Vivienne. vivienneyue@yahoo.com.cn
I worked with her on a smooth gown and it turned out very nice.

I always order the dresses without stones, as I don't care for the patterns they tend to create with the stones. I also prefer to use Swarovski or Czech stones instead of the Koreans. The Korean stones just don't have any sparkle.

Good luck!!