Do you have a matching McMaster-Carr part number? Their catalog is intimidating.
As far as hardware stores (Ace, etc.), I checked my local ones at lunch today, and found some grooved grommets that are much thicker, they look like they have the correct inner groove diameter, and a slightly smaller hole diameter. I bought a couple, but haven't tried them yet, and figure they'll be tougher to put on because of the thickness. But maybe they'll stay on longer because of that (??) Also, see previous comment that DMZ posted regarding breaking off the plastic push pin... so with smaller hole and thicker grommet, I'll have to be careful about that.
By the way, DMZ, thanks for the suggestion, but I would still like to use the "push-pin" design if I can. Those parts still seem quite sturdy (bike is only 1 year old).