I have a complete toilet with new slide seals, tank-to-seat gasket, and floor gasket for sale if anyone is interested. It looks brand new inside.
Otherwise, Amazon seems to have your parts. And most are even on sale!
Thetford 09872 Slide-EZ Valve Repair Package - All Plastic
https://a.co/d/dUSMCwS
-OR-
Thetford 12836 Electra Magic Slide-EZ Valve
https://a.co/d/1eiAi3E
Thetford 33364 Flange Seal
https://a.co/d/aLrNDwj
Like Bill said, the clips are all but impossible to find. But they are Thetford part number 36966 or 08637. I may or may not have an extra one. I’ll have to dig through the pile and see… I really need to inventory my stock!