Is this what you are looking for?

Numrich Gun Parts M&P 15-22

Unfortunately they aren't shipping anything right now due to the C-19 virus lockdown.

You are looking for the pin that holds the firing pin and guide rod post? I think that the one I linked to is the right one, but not sure.

Edit: Same pin at Midwest Gun Works. They appear to be shipping, but part is sold out right now.

I just did an internet search for M&P 15-22 parts to find both sites.