R12 has been pretty much phased out. It's also very expensive when you do find it, and you have to be licensed with MACS to purchase it. From what I've seen, you can use the substitutes as a drop in. Besides, you most likely need to replace the O-rings and other seals anyway, so why not evacuate it and do the conversion? It'll be cheaper in the long run than trying to hunt down R12.
