River wears this particular dress in a few episodes, normally with a crocheted vest.
More pictures after the break...
Above is from Jaynestown. One more here:
The next one is from Bushwhacked, without the vest.
And these last couple are from Objects in Space:
Some promo images:
Thanks to Moara for pointing out these behind the scenes pics:
My Photobucket Album for River.
Serenifly Costumes site for River.