R12 is no longer manufactured, but there's still plenty of it around for small systems. You can find people selling it on eBay, there are listings there as I type this for single cans at $13.00 plus $7.00 shipping.
As ShockT mentioned, putting 134a in would work, but you'd still have to evacuate the system first. BUT, if you still have a partial charge in the unit, simply replenishing it with R12 should buy you some time. You SHOULD find and repair
the leak, but that may be difficult adn or expensive. Heat-Seal Stik and Highside Red Epoxy
will both do the job, but they're not cheap, and you won't find them at HD. But there's a good chance that your leak is so small that $20 worth of R12 could keep it going for years. I once spent hours with an electronic CFC detector trying to find the leak on a buddy's old kegerator. Never could find it. Put in a few ounces of R12, one time, and it ran another 6 years til he got a new one.