Kragens, Napa and Auto Zone sell air conditioning system recharge kits. It is normal for air conditioning systems to loose pressure over time but it is usually due to a leak in the system that must first be fixed before just simply recharging the system. Air conditioning works on high pressure only a small leak quickly causes a drop in pressure. It is often hard to locate air conditioning refrigerant leaks. There is a refrigerant that has dye in it that helps locate a leak. After recharging the air conditioning with the dye if the system leaks it leaves a stain to help locate the source of the leak. An electronic "sniffer" can also be used to locate leaks, it acts like a geiger counter as you come closer to the area of the leak the "sniffer" beeps loudly.In either either case the system will need to be evacuated have the leak repaired and recharged with refrigerant and the proper lubricant or the system may soon loose its refrigerant charge again.Do you hear the air conditioning compressor switch on (with the engine running) when you turn on the air conditioning from inside the car. Get a friend to turn on and off the air conditioning while you listen with the hood open engine running you should hear a clicking sound on and off each time the a/c is cycled on and off or the a/c clutch is not locking up the compressor. This may be due to many things (bad clutch, bad compressor, no power to controls or compressor clutch) but it could be a sign of low charge the system has a low pressure switch which prevents the system from being energized if the system has low refrigerant charge. By protecting itself it prevent catastrophic failure of the compressor and ensuing damage to the rest of the a/c system if the system looses it charge. You will need to have the system checked out by someone experienced in air conditioning work who will attach pressure gauges to your car and see if the refrigerant pressures are OK.