Where can I get a cute dress for an eleven year for a wedding?

My little sister and I are going to an evening wedding at the Ritz Carlton in September. I have found a dress, but she does not have one yet. She is eleven years old and wants a dress that is not to babyish, puffy, or frilly. Since it is for a wedding it should not be red, black, or white. Preferably the dress should be a darker color because it is an evening wedding. My sister is a size 10 and pretty thin. If you could include links to dresses or names of websites I could look I would be very thankful.