Are you sure you need to replace the grommet? There's a pawl on the back side of the b-pillar that is easy to break off if you aren't careful. It inserts into the grommet.
If that grommet is what you need, I guessing you won't find it at an auto parts store. It looks like what you need is identified as 78100H in this diagram.
https://parts.nissanusa.com/p/Nissan...848-35F00.html