I'm putting together a 1979-81 WS6 rear disk brake set up for my 1978 WS6 Trans Am. I elected to buy the backing plates and power booster used, and complete the system with new or rebuilt parts.
You can buy calipers at NAPA. Get the ones with hardware (caliper mounting pins and dampening springs) you need this anyway.
Emergency brake cables can also be bought at NAPA. They cost about a third what they did from GM before they were discontinued, and a lot less than NOS (I paid $14 for one side and $18 for the other).
Scarebird Mechanical makes an emergency brake bracket and lever kit that uses Cadillac springs that are still available - about $75 - $100.
If you are buying rebuilt calipers, check them before you leave the store. The casting number of both sides is "451" and both sides have an "R" cast into them. Neither has an "L". The castings appear to be the same on both sides, but the difference is in the piston and / or ratchet mechanism for the emergency brake. There is an "L" stamped into the end of the bolt that the emergency brake lever attaches to on the left side, and an "R" stamped in the same place on the other. The "L" is located in the spot that is painted yellow in the first picture.
I was given a caliper that had "450" and "L" on it by NAPA. It was wrong - it couldn't have worked because it was backwards (the bleeder valve would have been on the bottom, and there wasn't room for the spring for the emergency brake). After a bunch of research, I found that it was from a 1979 Cadillac Fleetwood. I took it back and got one with the "451" cast into it.
So: always look for casting 451 for both sides, don't be confused by the "R" on the left caliper casting, and look for the little "L" or "R" stamped into the bolt