If I'm reading this correctly, hardrain is referring to the rubber seal for the airlock, not the lid on the primary. And since he only has one such rubber seal, which is currently, buried in trub at the bottom of his other primary, he's in a bind (apparently, he has two airlocks, but just one rubber stopper????).
Am I reading you correctly, hardrain?
If so, the answer to the question, "can I buy those things at the brew shop?" is yes, you should be able. to.