The same issue came up around here a few years ago, but the dive shop ultimately came up with a waiver/disclaimer stating that I was using the tanks/air for my own purposes. The idea of requiring a dive license has no legal basis- a dive course is hardly a course in pressure vessels and gases and anyone can purchase compressed, toxic gases from industrial suppliers (Co2, nitrogen, oxygen, acetylene, etc. etc.). Maybe talk to a gas supplier as well and see if they can help.