As said chap i'm asking what marker you are using as most elect markers won't use co2 as it freezes the electrics and knakers the orings
Co2 is cheap but... Depending on weather you need a expansion pipe/champer in your marker to stop marker freezing\throwing liquid out (if its a heat wave you don't need this which is never in the uk lol).
TBH just buy a steel £35 hpa 3000psi. When you get a regular paintball site they will alow you to fill for free even if you turn up on a punter day. Just don't abuse it as it affects us all
Sorry i'm drunk but thought i would help the best i can