I used to use a paintball canister but the refills were more expensive and the last time they overfilled it and that is not good (pretty scary actually), paintball places can be hard to find as well.
Now I used something MUCH bigger and pay $25 a year here http://www.airgas.com/browse/categor...183&WT.svl=183, it is done by professionals, tanks are exchanged and re-certified on a regular basis, the initial cost first thank and regulator valve (allowing you to regulate the co2) was $100.