They have meters to test ppm. The paintball CO2 tank you saw is a CO2 tank just in a smaller size. Because of how small it is, I'd suggest you not getting one of those and instead getting a scuba sized tank. You can probably find places to fill your tanks around local stores. Paintball stores, usually a sporting goods store, and maybe a hydroponics store. Hell, even home depot or Lowes might fill them (haven't checked). Goodluck.