I think the problem with normal air tanks is the volatility... CO2 is only at 1800 psi, where air can be 4500 psi. CO2 bottles also have a relieve valve, and it will not shoot across the room if it falls over.
I have Powertanks, and really like them, they have been very useful. I would recommend just purchasing one of them. There are a few knock-offs, and I think that they work well. A few people have cobbled together a CO2 system by getting a used CO2 tank from a restaurant supply or a welding shop, then buying a regulator off eBay. The Powertank regulator, in my opinion, is far superior and provides up to 200 psi, and does not freeze up.