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.