R12 is very much for sale. Just need a 609(MVAC) cert. A 608 (HVAC) cert. will allow you to buy it also.
R 134a is the final product that is for sale. R134 is just an easy way to refer to it and may or may not have been a "beta" molecule for R134a. (who knows what Dupont was doing then).
You've already got gauges; get the 609 cert. online (easy) and have a shop pull the vacuum or rent/buy a vacuum pump. You can pressure test for leaks (static) with R134a (but the you would have to have it evacuated) or use Hydrocarbon refrig.(evacuate to your BBQ) for leak test. Then change receiver/dryer, evaluate oil supply based on size of leak(or flush and start fresh), pull vacuum and recharge with R12.
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)