Rural Kings sells R22 replacement. It is made from propane instead of methane. Some people claim you can mix the two. R22 replacement requires less freon. I think one can still buy 30# of the real 22 for $450 ? +++? $15/#.
If you have a heat pump, 18 years is long enough. If you only use your a/c a few months a year, then 18 is still young. I've gotten over 30 years out of an a/c. Need to clean condensor, oil fan motors, make sure suction line isn't freezing up, some condensate on suction line. I would run it until it dies completely. Then find an a/c with the highest seer. Some may want to sell you the older 22 systems cheaper, be careful, R22 was taken out of production in 2010, only going to get more expensive.