Few things to try:
1) A SCUBA dive store, cause all you need is air, and that's what they sell! SCUBA tanks are pressurized to 2500psi. Tanks are cheap and available, Fills are not too expensive, heck, you can buy your own compressor easy enough.
2) A Welding supply store, they can definitely sell you a tank of CO2. Or Argon. Or oxygen. Or Acetylene.....
It is illegal (and downright dumb) to carry an explosive gas, such as acetylene, in an enclosed passenger vehicle.