JC - you will be best served by just finding someone with some r12 that can add what is needed. I have resolved two vehicles a/c issues over the past few days (a 1990 Buick Roadmaster and a 1985 Nissan truck). Both of them just needed a little r12 (approximately 1#) to bring them back to clear the siteglass and make 40dF duct discharge temps. There is nothing like having a working a/c in this part of the country during the month of August. Both owners were tickled pink
Unfortunately, if they had taken them to an a/c shop they would have tried to sell them a "conversion" for $600+ when all it needed was a little r12.
I would stay away from anything but r12.