[Drake]

Firstly, as ccyg8774, HFC134a (aka "duster" and "blue gas") is not the same as Green Gas/propane: its much lower pressure. You'll usually find this for Japanese guns, specially older models, and some cannot support the higher pressures of green gas (they'll kick like a mule but you'll crack your slide or damage other parts, or blow out reservoir seals etc)

HFC134a (tetrafluoroethane) is a refrigerant which was also used in duster gas, but some morons started huffing it and died (literally) and its use is also restricted because of climate change, so most duster gas now is difluoroethane (HFC152a) which is unfortunately lower pressure and doesn't work very well to power airsoft. However HFC152a is more flammable than HFC134a so duster for specialized equipment (such as professional photography) can still be found, which still uses HFC134a. Expect to pay considerably more for it than your average Staples duster.

At this point in time, unless you're a collector or have your heart set on some unusual gun only produced by some rarer Japanese company -- which absolutely cannot run anything else but duster -- I'd suggest you just pick from the list of commonly available guns which can run propane. Anything coming out of China/Hong Kong/Taiwan runs propane.