The Home Depot in my neighborhood sells them. Four to a package, less than two bucks.
You can probably come up with one...and if not somebody can turn you one on a lathe. I chime in only because I had a white bumper on a cue and I can tell you that they don't stay white very long. It will look very dirty very fast. You may want to consider that.
I understand that it would get dirty over time. But I still think it would look cool. If anyone has one I would love to buy a couple. For that matter if anyone has a brown bumper I would like to buy a couple of those as well.
Is there any type of solution that could be applied to a fresh bumper, almost acting as a seal or finish, to protect it from discoloration without making the rubber too hard or brittle?