Milwaukee is popular for Co2 within the US. It's the cheapest reliable thing I've seen, though I'm not sure they make a paintball sized reg. Azoo has worked well for me, though the needle valve is touchy. I'm fairly sure they have something in a paintball sized threading. Many others have been used, and I have yet to hear complaints about the major brands. The main thing is making sure you've got a solenoid and a good timer or two; nobody has a perfect memory, and leaving CO2 on over night can cause more damage than forgetting to turn the lights out.
-Philosophos
