Welding supply stores and home brewery supply stores are good places to start for tanks. Larger cities will also have scientific/medical gas places as well. Sometimes you "rent" the tank, sometimes buy it. Just depends on the local setup.
i dont think you can put co2 in an o2 tank, because no one will fill it.
all tanks air, o2, co2, n2, nox, all have to be labeled and visually inspected and certified from time to time to make sure they are still good. (short version of explanation, use search for the long version)
you cant have somebody fill an o2 tank with co2 because of the vendors implied liability.
i had a co2 tank leftover from the last time i brewed beer at home. i use the co2 tank from my kegerator, so i just go to homebrew supply joint and get refills, and when its time to switch bottles i just trade it in for a newly certified bottle. its pretty damn cheap.
the regulator is only used to reduce the preasure of the gas to a useable volume. otherwise you will have issues.
a co2 tank is putting out a volume of presure that would freak you out.
you can get the bottle from just about anywhere. the regulator from a party supply/welding supply/scuba shop/party supply store, the tubing and 1/4 inch fittings, and air chuck or whatever at the depot.
The various compressed gas cylinders take regulators with different fitting sizes and pressure specs. There are only a couple that are interchangeable. You don't want to be careless around these things because you can get hurt bad . . . those things can turn in to a missle, bomb or you can frost bite or freeze body parts!!!!
This sounds like a cheap easy way to have oba without having to pay for the Powertank name. It would probably be a better idea to get an adjustable regulator, right? How long will a 5 or 10 lb bottle last running air tools though?
Is this a better setup for airing tires up after running trails, more than running air tools?