I agree with Chad. Converting over can be tricky and will require charging the system more often. Most of the problems with R-134 are "leak-based" due to it's smaller molecular composition. In addition, R-134 conversions tend to be less efficient than R-12, especially in stop and go traffic. It can be done however.

The main reason that R-12 is difficult to find is relates back to the EPA's 1995 ban on its production.

But hey, I am quite sure that Raymond will give another extremely thought provoking response and say "just use Napa"... Of course he won't supply a part number... but hey, "it's in the parts manual" !