You can go to any reliable (good) automobile air conditioning repair facility, they can be hard to find just as a good dealer service department is. Does your a/c not perform because it is low on refrigerant? if its low, it has a leak and must be fixed before a recharge can be done, If you can afford it stay with R12, (you can still buy it) and should still come out less than what most places will charge to change over to R134a and will work much better especially in warmer climates.