1986 Mercedes 560SEL-Air condditioning?

Freon 12 can still be bought - legally. It's not cheap but the cost of R134a isn't cheap anymore either. Professional A/C shops will have all the Freon that you need.

You can still get R-12, the manufacture of new freon in the US is no longer legal but there are still supplies. It is very expensive to fill with R-12, you can use alternatives such as R-406a. If it needs freon its because the freon leaked out from it. You need to figure out where the leak is else a recharge is just an expensive waste of money.

Retrofitting with R134a is painless, so you can go this route too. Usually when you lose freon, you got a leak somewhere... so it really depends on how much you lost... your system only has about 3.2lb of freon... so if you just lost 1 lb, then it is better to just refill... if you lost more, then you got to find the leak.