I think it is pretty much just closed cell foam like that on the trekking bars, but with a smooth outer layer. You could probably ask at a hardware, plumbing supply, or sporting goods store. The closed cell foam that is on the trekking bars is what I have on mine and I do not find it to be too bad in the rain. It's closed cell, so it doesn't absorb water like a sponge and dries fairly quickly.

EDIT: OK, here's some pipe insulation that is the same but brown, and also too thick, but it gives you an idea that plumbing supply stores might be able to source it. And based on this page, I think it is called "rubber foam tubing insulation." And again, it is used in plumbing, heating, AC and that sort of thing. Looks like this place carries it in 3/8" thickness and almost certainly a plumbing supply can order some (maybe in larger quantities than needed though). Maybe Home Depot or a similar place has it on hand by the foot.