Doug,
I bought some rubber like this off Ebay.
http://www.ebay.co.uk/itm/4mm-thick-neoprene-rubber-sheet-black-1mtr-x-1-4mtrs-/170837289394?pt=LH_DefaultDomain_3&hash=item27c6b229b2Then cut out two large rubber rings, then two smaller one and made my own grommet by glueing them together using some serious super-glue.
It was far easier than hunting an OEM or similar item down and I got the eact shape I needed
PS - If you don't fit this grommet you'll get a lot more road noise inside....... guess how I know that !