I got the BikeNashbar second wind "clone" for like $7 after some crazy sale with free shipping awhile back:
http://www.nashbar.com/profile.cfm?c...All%20Products
It's nice because it can use 12g unthreaded with it's own screw-in cup, or any size threaded if you just the the headpiece.

The hand pump is not so easy to use when you get to psi beyond 40 but it's functional for emergencies.

Small enough but not tiny by any means.