R-12 is not as expensive as it used to be and can be found at service facilities without looking too hard. You need a license to purchase it, however, so you won't find it on store shelves.
If you just need a top-off, spend the extra few dollars and have it done with R-12. Major repairs, on the other hand, are another matter.
Upgrading to R-134a is doable, but I doubt you'd be satisfied with the results. It flat out won't give you the sort of cooling you're used to.
I used one of the R-12 replacement refrigerants as a quick fix (a top-off) and it worked well on another car of mine. Some question its long term reliability. I didn't keep the car long enough to find out.
Furthermore, I *think* the R-12 replacements are flammable; I'm not happy about that.