Not sure if you ever found a replacement but I guess you could either make one out of a sheet of neoprene rubber of a little less than the thickness as the others because neoprene might not compress as much. Or you could use one of those gaskets used on headlamp bulbs, look very similar. Either way, I hope you find what you need because losing that part can be so annoying with a lose plug coil.
You might also get away with cutting one out of a firmer foam, the ones that are sold at walmart as interlocking ones. The idea is to make sure that there is a bit of resistance when you tighten the bold holding the coil down, plus prevent dirt from falling into the plug well.