Bought a used kit from a member on here last year but didn't get the rubber weather seal grommet piece for the valve cover. Does anyone have information on what to use here?

 

Gonna need more info, I’ve built dozens of these engines and have no idea what you’re talking about. Valve cover gasket? PCV grommet? CAS o-ring?

what engine, and maybe a pic of what you’re talking about goes.

 

Sounds like PCV grommet. Available at most auto parts stores, fairly cheap.