R-12 is most assuredly still for sale in the 'States. I priced it this summer when doing some a/c work. Just about every McParts store carries the small blow off cans, if not the larger 30lb containers. Last I checked the price was running $25-$30/can. You must have a 'section 609' mobile a/c certification to purchase R-12. I've had mine for almost 10 years. The certification can be gotten online - I think the URL is http://www.epatest.com
R-12 has not been manufactured in the U.S. for several years, but there is still plenty available for sale.
I don't like the conversion kits for a number of reasons. To properly convert from R-12 to R-134 the system should be flushed to remove the old mineral oil used to lubricate the compressor. Mineral oil is not miscible in R-134a, hence just collects in low spots in the system. Too much oil also lowers performance - a bad thing considering that R-134a doesn't usually cool as well as R-12. Lastly, different compressors require different viscosity oil. Some like ISO 42, others ISO 68, and yet others ISO 100. Just dropping in the POE oil charge which comes in the death kits may or may not be sufficient to keep the compressor alive.
I think your best bet is to scare up some R-12 and just recharge. The certification can be had for ~$20 or so, then go look on ebay for R-12; it's likely to be getting cheap this time of year.