Many of the slingshot makers out there make rigs perfect for BB's, but the only thing you really need to worry about is your bands. Simpleshot sells BB bands and tubes, so you can always throw those on an existing frame and shoot BB's well. You just don't want to use bands that overpower your ammo and make you inaccurate. Common setups I like are single 1632 tubing or #64 office rubber-bands. As for frames, I've seen some pretty cool BB sized frames at the following company websites:
WASP Slingshots - Imp
Simple Shot - Deadringer and Jelly Bean
A+ Slingshots - BB Shooter, The Wasp, The Dandy
Pocket Predator - several small frames that would be great for BB's
and I'd be remiss if I didn't mention JoeyJ5Lujan's LBS frame. He's a member on the forum, and if you PM him he can quote you a price and give you some info. Good luck and welcome to the forum!