Go to the nearest auto parts store and get a can of R134a and a re-charging kit. The kit has the adapters and valves needed to charge the system. [
B]Read[/Bs the instructions 2x before starting. It gets pretty wasteful if you screw up and the refrigerant escapes. Oh yeah, it's extremely cold as well so wear eye protection and gloves. DON'T ask me how I know.
Depending on how low your system has gottn, it could take up to 2 cans. They also offer a sealant mixed with the R134a so get that if you can.
Sure, repairing the system properly is the right thing but once you have the kit you can add a can or 2 each time you need to, forever.
Bam!!
http://www.aa1car.com/library/ac_recharging.htm