Allow me to save you an assload of time, Oreilly / Kragen, Autozone, and Napa Auto Parts all show that they have reman Master Brake Cylinders. The problem is, they don't have any of them in stock and if you have them do a check to get one, they will tell you it's been discontinued. There is ONE that Oreilly shows and has, I ordered it and sadly the bolt mounts were horizontal not vertical so it was useless to me. (It was the Cardone brand) I did A LOT of searching and asking around and it seems like the only financially feasible options are..
1. Go to a salvage yard around here and pull a used one, or ask around on the forums since some people do it to have extra for this exact reason.
2. Buy a rebuild kit, didn't really find any ones that I was absolutely sure were going to work / be the right one. (Could be a potential 40-50$ dive)
3. The most expensive is to buy a new one, places that I saw have them that range around 130-150$
If I am wrong, feel free to correct me but I spent probably 8 hours calling and searching for information about reman Master Brake Cylinders for 79-81 280ZX's.