Might give Marine Sanitation (marinesan.com) a call.
As recently (?) as the 2020 Seattle Boat Show, they still had rebuild kits for the WC HeadMate. I picked one up as a "spare". Prior to that, Downwind Marine (now called San Diego Marine Exchange) had the rebuild kits from time to time.
A more interesting alternative might be a replacement pump. In talking with Clay at Marine Sanitation, the Raritan PH-II pump is compatible with the Headmate head, almost a bolt-on (he says the bolt pattern "almost" matches, the mis-match can be addressed by using the next smaller size fasteners, or by opening up the bolt holes a smidge).
Raritan Compact II Kit, PHII PHIIRFK
marinesan.com
A little spendy but, I'm told, a MUCH better pump than the WC, and parts are readily available.
I haven't replaced my pump yet, but as soon as I run out of rebuild kits I'll probably go that way.