River wears a pink dress with a brown one-sleeved shirt and a crocheted overlay when we see her help out her crew during the heist at the beginning of Serenity. She wears just the pink dress at the end of the movie.
More after the break...
Some more promo pictures:
And some screencaps:
This costume was at the Propstore for a little bit, in a few forms:
My Photobucket Album for River.
Serenifly Costumes site for River.