Any grommet of roughly the same size but a little larger will work. Take a razor and slice one side of the grommet then wrap it in the hole until it over laps, mark it and cut to size. When installing it put the cut on the bottom of the hole. Most hardware stores will have them.
Another tip from an old roadie/ham radio guy: many times you can improvise a solution from what you have on hand.
I HAD bought a grommet for the battery box hole for the 200A alternator cable, couldn't find it. Had some 1/4" ID fuel line handy (with internal braiding), so I cut a length of it about 3" long (C=Pi*r^2 or something), split it along ONE side lengthwise (easy enough to do if you immobilize it in a vise to get the line straight then remove and follow through), then wrapped it around inside the hole. Didn't think to glue the ends together.
Will work for now, or until I get around to ordering another of the correct sized grommets, but I suspect those are as likely to wear/abrade as any others. A spot of silicone sealant holds things in place for now.