For those who asked about this recently. It's just a standard 0-ring from Pep Boys auto parts store. The easiest way to get the right size is to ride the bike there, or bring your bar end with you. I forgot to note the size I used. You want a nice fat o-ring that is fairly tight on the bar end. I put a zip tie around it and cut off the end, leaving a little tab to make it easier to slide it out of the space between the grip and the bar end while riding.