R12 is out there for sale. Current pricing is about $2000 for a 30lb cylinder. I know where I could lay hands on about 120lb of the stuff for the price of asking. It's not as rare as you might think.
As for filling R134 - it's done by weight. When that isn't appropriate or possible it's done by system pressure which varies based on ambient conditions. It's not that difficult really.
Being a shop owner I invested in a $4,000 AC machine so I can have my joe's recharge systems and be fully compliant with the laws of the land. Don't wish to engage in any imperial entanglements over not properly recovering refrigerants.
GD