What is it and where can I get one?
 

This valve(?) threads into the intake manifold. Part of the vacuum system.

I broke the plastic nipple. Hard to find a replacement when you have no idea what it is. See attached pictures.

I had the same question yesterday (noticed the hose attached to it was literally falling apart.) It's the altitude compensator, apparently. See this thread: https://www.rx7club.com/1st-generati...nifold-891695/

I haven't tried taking the very weak hose off of mine yet, hopefully I don't break it in the process. You can get a replacement from mazda for like $50 or plug it, apparently it's not terribly important if you don't drive over mountains too often.

5280 feet
 

Guess where I live? Denver.

if the nipple does break off, it can be epoxied successfully.

The thing acts a little like a PCV valve. You can remove the plastic part and the guts inside it, install a piece of 5/8 heater hose, purchase a PCV valve such as Fram part number FV333 or FV266, push it inside the hose, and you regain the spring loaded valve action of the altitude compensator/PCV valve. I've done this and it works.