I looked into it, and IMO it was not worth the trouble to set it up. You need the CO2 bottle, then you probably need a "remote hose" or some other way to get a generic connector onto the special paintball fitting that is on those tanks. Then you need some more fittings to get a regulator on it. After the hassle, you might save $10 over going to a welding supplier and getting a 5 or 10 lb tank already filled. I called all around here and found a place that would give me a 5 lb tank filled for $65. If you figure 25 for a paintball tank, 25 for a hose, (ebay prices) and 5 for fittings, you are not saving enough IMO.
I've also heard that the co2 may have some oil or other contaminants, but I'm not sure if I believe it. I would call some welding and/or medical gas suppliers and ask them if I were worried about it. But again, doesn't seem worth the trouble.