Joining the search engine for good backdrops:
http://www.photekusa.com/

Click on Backgrounds-in-a-bag, and then at the very bottom, click on Colors.

I've got the Stormy Grey one, and basically just use a colored gel on the light that hits the backdrop. Blue, red, orange, yellow - whatever - and you have a "cloudy" version of that color. The reason you keep it in the bag is to keep it "ruffled" up - you don't fold it, roll it - you stuff it in there as harsh as you feel like shoving, and then zip it closed.

good luck - hope that helps-

-Lew ;-)