Here is a link to where I bought valves for ours:
https://www.amazon.com/Valterra-TC37...4J4Q8G64JZWX22
I replaced both my gray and black tank valves with these sand they both operate effortlessly compared to what came on the rig originally. It is not a difficult job, but will take a little time. If you can have someone help you it would be nice, but it can be done by one person.
Jack