We use to use the old crep soles from shoes. Espadrils come to mind. They are rubbery and have bumps on them. I haven’t seen them in years so try garage sales and flea markets. The shoes date back to the 70’s and 80’s
I use the crepe rubber bars and pads for cleaning my sanding drums and discs. but when they really get gummed up I spray them with easy off oven cleaner and let them soak for a couple hours then scrub them with a brass brush and rinse them off with the water hose. Get about 10 times the use out of them.
When I can't find my rubber belt cleaner, I can get at least some of the clog off by sanding on the end grain of a wide-grain wood that doesn't have a lot of resin in it, like newer fir. I've also tried hard-natural bristle scrub brushes, short bristles, and that has worked a little, got the worst of the resinous stuff from sanding old-growth yellow pine off . Nylon or other synthetic bristles melt and make it even worse.
Meanwhile, try asking at Kapaa Ace Hardware for the Ace 13795 Abrasive Belt Cleaning Stick . It's a standard Ace Hardware product, if they don't stock it they should be able to get it in fairly quick.
Soak 'em in lacquer thinner for a bit and thin wire brush. Been gettin' the gum out that way for many years. I do the same thing with 3M "Diskits" but soaked in acetone.