sarin's right as to the location of the low-side service port. as for getting your hands on R-12, you'll find it sitting right next to a triceratops

it's hard to find and extremely expensive, and if I'm not mistaken, you need a license to use it anyway (I could be wrong here.) your best bet is to get a R-134a conversion kit. I dunno about where you live, but at the auto zone in my area (which is where I work) the conversion kit is like $35. it comes with the new fittings, the necessary compressor oil, and some R-134a refrigerant. basically everything you'll need. and from what I've heard, it's really easy to use. one setback though is you'll need to get all the old R-12 out of the system first.... technically it's illegal to just vent it into the atmosphere, so I'm not telling you to do that... but I can tell you that the low-side uses a schrader valve like you'd see in a tire, so any tire valve stem remover will remove it. when the blazer needed the conversion, somehow this valve stem came out.....

(ps, anything's legal until you get caught.) after you get the old R-12 out, I think it's just a matter of adding the oil and refrigerant.
Dan