Where did you find the grommet?

My mechanic told me the antenna manufacturer would not sell the grommet separately from the entire antenna assembly.
My 85 300D's was damaged by the PO who apparently attempted to force the antenna to retract when it was stripped. Replacing the antenna was easy enough, but I ended up using black silicone form-a-gasket material to kludge a grommet. (If you coat your finger with liquid soap you can shape the silicone without it sticking to your finger.)

As I recall, the grommet was much larger on the underside, which would imply insertion from within.