This topic regularly rears its head on the DeLorean forums too, so here goes.
R12 is thoroughly illegal as a ozone-depleting CFC. Modern cars use R134a but this runs at a higher pressure, and a lot of places will try and sell you a conversion from R12 to R134a. A lot of the yanks are doing this on their DeLoreans and there's an excellent write-up here, and I believe Renault used the same AC compressors for years - the DeLorean's one is Renault sourced
http://www.dmcnews.com/Techsection/r134.html
HOWEVER the reason so many poeple stateside are converting is because R12, though legal, is extortionately expensive, and bizarrely, the drop-in R12 replacements are illegal! Not so over here. The problem you get is an AC place will try and sell you a conversion than simply re-fill with replacement because they have to buy a big bottle of it and won't make anything like as much money. As you can see from the above article, it's nowhere near as simple as just dumping out the old gas, running a "conversion oil" through it, and re-filling.
So.... my barometer for AC places is to ring up and ask "do you do drop-in R12 replacement" and if they say no - get it converted, go use someone else!!! There're a lot of different brands out there for basically the same thing -
R413
R416
RS24
Isceon 49
All of these have been shown to work in the DeLorean's R12 system and a vac, leak check and re-gas typically costs £50-75. They are all perfectly legal and will combine with any residual R12 in the system with no problems.
R12a is largely butane!!!! so yes, slightly flammable, and available in the US usually as a dodgy mexican import. Here the EU has stricter controls.