Check out dance wear at resale shops. There are usually sparkly leotards and other things. Or even try a dancewear/costume shop. I think there's on in the Grapevine Mills mall. The only other one I'm familiar with is Dance World in Denton, next to the Kroger on University Dr.
Also, you might turn this into a craft opportunity. Get a T-shirt from Michael's or Hobby Lobby, get some fun fabric paint, and let her make hand prints all over the shirt. You can write her birthdate on it, "Happy Birthday" or "Birthday Girl" or anything like that, maybe even pick up some "jewels" that you can stick to the shirt using either the paint or some fabric glue.
We did this for Valentine's Day and the kids had a blast and still use the shirts to sleep in.