You can't get R12 over the counter any more.
You can replace R12 with other refrigerants but it may not work in your unit. A drawback is that you need to have your unit evacuated before doing so, which means taking to a shop. Then replacing the R12 with a replacement refrigerant like R12a or R22 which may not work as the seals and pump in your unit may need to be replaced.
Some shops can still get R12, but they will not sell to a consumer directly. It must be installed by them.
If you can add refrigerant, you could attempt to cheat and simply purchase a Red Tek 12a kit but in order to use it properly you need to know the specifications of the unit and how much R12 it is designed to hold. This might destroy your unit, do so at your own risk.