I had a paintball retail store and used CO2 tanks to refill the smaller tanks for the markers.
I have to disagree that the air is clean, we found the filters on the higher end markers would clog after about a season of use with CO2.
I would also be concerned about passing liquid CO2 thru my gun. If I were to go that route I would put something to capture any liquid (we called them expansion chambers on the paintball markers), then also get a good regulator as CO2 tanks will vary in pressure as the amount decreases and the temp of the tank changes.
Refilling a 50lb tank use to cost me about $9, and then the tank rental was $50 a year, might be more if you only have one tank, I had 10 in the store.
You might also consider a scuba pony tank or a used scuba tank, that air is filtered before filling the tank. The pressure is much higher than CO2 (usually about 3000 psi for a standard tank), but a good regulator should put you down to where you want to run the gun.