i hope you are typing about a v6 with a r-12 freon a/c system.
there are some petrochemical substitutes available for a reasonable price. duracool, redteck, and freeze. others may degrade the system components. r-134a will plug your system if you don't flush and dry the r-12 system, before using.
doing a proper r-134a conversion, is more than putting on new fill valves. the condenser, compressor, pitot tube and drier needs to be replaced. doing that yourself will save you some money, but be sure to flush the hoses out completely. any remaining mineral oil will solidify.
if you want to use real r-12. you have to be licensed to buy it. it will cost you over $30/oz.
you might find it on craigs list or eBay.