If it's just a little low, you could get a Merc specialist or an A/C shop to add some Freon R-12 to the low-pressure port. If it's discharged, you need to find the leak and fix it. Then evacuate to a hard vacuum and recharge with R-12.
R-12 is no longer available to the public and only a certified technician can handle it. You can't buy it yourself legally.
If you want to find a specialist, here's our directory link for you: