You're looking at the Fuel Tank schematic, but I believe you should be looking at the Frame Cover schematic. There are two grommets used for this piece, part # 09320-08018 (CUSHION, FRONT). Good thing is they are less than $2.
Dude go to home depot or lowes (lowes, preferably, IMO) and look for anything round and rubber that is approximately the same shape. Whittle it down a bit with a knife, cut a groove around the circumference and drill a hole through the middle and you have your grommet. Or just hunt through a selection of grommets at a nice hardware or automotive store until you find the right one.