To only compound the problem, you're not just looking for a gasoline resistant grommet, but an alcohol resistant one as well due to the ethanol content in today's fuels ... wish I could help you, but I'm suffering from similar problems - as are my marine technician friends & many others I'm sure
Aircraft Spruce & Specialty is a helpful company I called a few times when I was playing with ultralights, they've alcohol resistant fuel line readily available: BING ALCOHOL RESISTANT<br>FUEL LINE from Aircraft Spruce
and as many ultralights and experimental aircraft use plastic tanks and grommets they may well have what you're looking for - just not in their online catalog... I'd give them a call.
As anything for "aviation" is subject to a serious price markup even if not FAA approved, get a part number & shop around