New day New question.

I think I need to replace the breach seal on my Benjamin Rushmore. It is the one on the end of the barrel , where you put the pellet. I seem to be having trouble getting what I need. I ordered some twice and have gotten the wrong thing. Am I calling it right or is there another name for it?

Where do you all order breach seals?. Is it possible that the big box stores would have something like this? It seems like all I do is wait on the mail lady..

Thanks in advance

Bobby