SMS - get your 609 certification - costs about $20-$25. Once you have that, you can buy your own R-12.
As Larry posted elsewhere today, R-12 is getting cheaper. It's ALOT cheaper than retrofitting an R-12 system for R-134 which in my opinion is not very effective in a car that was designed for R-12.
After you have the juice in had, search around the archives here for tips on "hot-wiring" the pressure switch on the receiver dryer. When a system is bone dry, it will not engage. You hot wire it to charge it up and possibly add a little dye while doing this to help locate the leak.
I'm not suggesting that a couple of cans of R-12 is cheap. I am saying it's cheaper than most any other approach and as Larry pointed out, you don't end up with a "mix".
My 2 cents.
PS - Kinda strange that we're talking about R-12 in January?