Anyone know what this or how to get a new one?
frank, there is no picture for us to look at.
tom
Your picture is a spool valve. They are readily available and many threads on this site about suppliers and rebuilding as well.
Try a search and you’ll find part numbers , prices, and vendors..
Search for air dump valve
very common failure. i have replaced the one for the sewer dump and for one of my interior air doors
tom