Here's what I know.
1.) You need an EPA 609 license to legally purchase ANY refrigerant other than R134 a.
2.) The EPA 609 certification is an open book test that you take online, the cost is about $20. You can download the study guide for free. You can use the study guide while you take the test: the rules state this is OK. See
http://www.epatest.com/609/openbook/ and
http://www.epatest.com/609/openbook/. Guess what? If you get the PDF doc study guide, PDF documents are searchable using the search feature. So if you can read, the test is very simple. I still have a copy of the study guide in PDF format that I am willing to share for free.
3.) There are subsitutes for R12 that are not expensive, but you still need the EPA 609 certification to purchase them. Some are blends which require special considerations when recharging the system. At least one of the subistutes, R12a, is a highly flammable blend of propane and butane.
4.) R12 is still available in 12 oz cans IF you have a EPA 609 license and are willing to pay $70 a can for it. The 5.0 Mustangs take 3 cans, so that is $210 just for the R12. As always check Ebay. There are people claiming to have R12 available in 12 oz for less than the $70 a can price. As with any Ebay item, buyer beware.
The tools & procedures for conversion or recharge are described at
http://bellsouthpwp.net/l/r/lrichker...conversion.htm. The process described is for R134 conversion, but you can skip all the flushing and purging if you use R12.
4. More information on A/C repair is available at
http://www.autofrost.com/ and a google search,
http://www.google.com/search?q=R12+r...x=&startPage=1