The only acceptable alternative is R-134a and there can be a problem retrofitting it. Molecule size of R-12 is larger than R-134a. When using a new replacement R-12 a/c hose, R-134a can leak slowly through the walls. Existing hoses have a/c oil and crud filling the holes and will almost always not leak R-134a. Again, this possible problem can occur when replacing a hose which does not have the barrier system for R-134a.
Factoid, R-134a does not cool as well as R-12. Newer systems are designed to compensate for its less efficient properties.
That said, some years back I sucessfully converted by Buick to R-134a. I do NOT recommend it. R-12 is available. Federal tax makes it a few bucks more expensive. Last 30# can I bought was a horrible $300 and now it's about $800. But you only use 2 to 3 pounds so you might pay $50 extra for a recharge. Not anywhere near the cost of converting plus possible headaches.
If you wish to become an expert, there are sites wherein you can take an online course and the test for about $30 and you become Federally certified. This allows you to purchase R-12 legally. I have purchased R-134a without a licence.
My a/c guages were $30 at harbor freight a few years back and are handy to monitoring and adding freon to the system.
1993 MB190e 104k