Compressed air is much cheaper than CO2.
Use a compressed air tank such as one for scuba diving or for SCBA (Self contained breathing aparatus)
Fire depts use SCBA units - and upgrade from time to time. Most have gotten rid of or don't use the steel containers anymore and may have them lying around. You could get one for a donation, and for a donation/friendship with your local volunteer fire dept get it filled there as long as it is hydrotested. Your CO2 tank will need to be hydroed every (I think 5 or 7??) years too. I pay $15 to get the tanks hydroed.
Use an oxygen/CO2 regulator - adaptors can be bought at any good welding supply store.
Depending on the tank you could have between 2000 - 4500 lbs pressure in that tank.
That's what I do - but my donation is my time.
Dan
Member: Broadway Volunteer Fire Dept.