Just so you know...R-12 is no longer available on the open market. Good luck finding some.

R-134 may not be quite as good as R-12, but it doesn't destroy the environment the same way R-12 does and it's extremely cheap by comparison (mostly because it's still being made). Also, anywhere you can (leagally) buy R-12 from require that you have an A/C service license. Even then it's still $100/lb or more.

There is an R-12 substitute on the market that advertises itself as a form of R-12. Don't be fooled, it's not actually R-12. It does work a slight bit better than R-134, but it's still quite a bit more expensive.