Project X has some great information, check that thread:
http://forum.oc-forums.com/vb/showthread.php?s=&threadid=66635&perpage=30&pagenumber=1
I have looked for places selling air conditioning compressors, and didn't come up with many good places...
http://www.e-johnstonesupply.com
Well, in fact, I have yet to find ANY place that will sell retail, except just the compressor, and that tends to be like an appliance repair shop, one called tribles:
http://www.tribles.com/
and they sold retail
http://www.jneuhaus.com/appliance/
a bunch of listing of different appliance stores... the problem is that you have to know the part number that you are looking for, and it is hard to find a listing of refrigerator/air conditioner parts that have the specs that we are interested in...
As for which refrigerant is best, well, you can't get any except R-134a without an EPA liscense... you can get R-134a in the form of air conditioner recharging refrigerant from an auto parts store. I have not confirmed this, but seems logical.
R-12 has been outlawed, as have all CFCs. there still are HCFCs Hydrogenate ChloroflouroCarbons, which will be outlawed by 2020, and HFCs like R-134a are completely ozone friendly... unfortunately they are not as efficient...
R-134a will take you to -16*C max, R-12 will goto -29*C, R-504 or R-401a or some numbers like that will goto -60*C, Unfortunately you can't get those refrigerants unless you have some refrigeration guy do it, and you need special compressor for those low temp refrigerants...
However there is light at the end of the tunnel... Propane, yes propane... it is basically an ideal refrigerant, and we have talked alot about it in the Project X thread, so I will not repeat it, but it is the way I would go with if I did not have a refrigerator that I ripped up... Anyways, read project X, I'm damn tired from installing a keyboard slidy thingy on my computer structure... Best, Bill