a big truck stop would carry them. dont know what is around your area.
a pilot, petro, or a chrome shop would have them.
depends on what you put them on, but on most semi the mount another air tank and 1/2 line to feed them they will drain your tanks very fast.
if you dont watch what your doing it will pop off your brakes driving down the road.
i know a guy that crawled on top of a train that was parked over nite. basically stole them off the top.
wouldnt recommend that but