A possibility. If the grommet is located to the passenger's side of the RH firewall to shock tower brace (see 68 Mustang Electrical Assembly Manual AM0032 pg 76, ref E8-8650-3, item 16, 2 used, item 26 - hole dia, item 49 a confusion factor) it is Ford part number 380851-S for which no documentation has been located.
So, go to Lowes hardware. They have a selection of grommets. Also ACE Hardware "might" have something as well.
Jim