The average lifetime of a condensing unit is 10 years.
You will probably be unable to purchase one on your own. Federal law prohibits sales of CFC & HCFC refrigerants to non licensed individuals. Most condensers are sold prefilled with R-12.
You may have a freon leak, a restriction in your condensing or evaporating coils, a faulty condenser fan capacitor or motor, or a faulty compressor. You are better off calling a licensed HVAC mechanic to at least isolate your problem.