Now, I'm new to robotics, but I'm familiar with CO2 and HPA systems. I'd stay away from CO2 if I were you - as maddox says, it's a fickle thing. Your output pressure can change by ~150psi just by tilting the cylinder if you've got a big one.

I would recommend that you do your testing with a High Pressure Air (HPA) system or a small compressor. You can pick up new .08l/3000PSI bottles for around £40, and you can go to your local paintball site or scuba center to fill it. You can also get regulators for the HPA bottle fairly easily - although I'd stay away from Ebay ones for the time being, they tend to be fairly unbalanced with an output pressure of 800PSI - far more than what your parts can handle.