The last I checked, you could still get R-12 here in the US if you had a "green card" refrigeration certificate (available online), but the prices of R-12 were going up. I have not checked in a few years.
In automotive applications, I have had good luck replacing R-12 with R-134a. The cooling capacity was probably about 10-15% less after the change to R134a, but it still was good enough for use in the 120 degree Phoenix summertime.
Before changing the refrigerant, it was necessary to flush the system & remove all remnants of the old oil. R-134a is not chemically compatible with the R-12 lubricant. A lot of guys say that R-12 hoses are too permeable for use with the smaller molecule of R134a. While this seems to be true of new hoses, the old R-12 hoses, that have been in use for years, seem to be OK with it. I did not see a measurable loss of pressure 2 years after the change over. I used all the original hoses. The only parts I changed were the compressor, the receiver/drier, the connection ports & the O-rings.
I adjusted my charge by monitoring performance of the system & stopped adding refrigerant when my high side pressure increased with no improvement in cooling. My final charge weight was slightly less than the original specified R-12 charge.
I now find R134a for $5-$15 per 12oz can, depending on where I buy it. I just filled up the system in my (originally 134a) TDI with the cheap $5 stuff. My air blows at 38 degrees with the fan on full blast when it is 90 degrees ambient, here in humid Florida. I think that the cheap stuff is fine.
I see that your location is listed as Londontowne. If that is jolly old England, than you may have another option. I believe that in the UK, propane is commonly used as a refrigerant in automotive AC systems. Here in the US, that is not allowed.
