I've had good luck with Vintage Air and Hot Rod Air. Hot Rod Air seems a bit cheaper the last time I looked. I think it's vintage Air that has released a MARK IV under dash unit.

For my personal 50s cars I generally like a nice under the dash unit to kind of give the interior an era correct look. They also have a couple of advantages in that there are no holes to cut in the dash or tacky plastic under dash vents, and are pertty easy to hang.

I've picked up fairly nice units out of wrecking yards and swap meeets and seen some on E Bay for not too much. Usually a good cleaning and a bit of detailing is all that is required to put them back in service.

The rest of the pieces (condencer, receiver dryer, and compressor (I like the Sanden 508s)) I pick up locally. Compressor brackets can sometimes be a bit problamatic depending on the engine but there is usually something that can be modified a bit to work.

I have hoses made up locally.

I normally use Freeze12 which is a drop in replacement for R12 as it seems to work a bit better than R134 in the older systems.

One last thought when adding an AC system is that normally you will need to upgrade your cooling system also.