Need a rubber plug for something? Try wine "corks"!
Most newer ones are rubber, and come in handy for all sorts of jobs. Found a whole box at a thrift store, so I grabbed a handful. Installed one on the back porch door as a door stop to prevent the door from hitting the brick wall. Works great! And if it eventually wears out, cheap to replace!
Also could work on the end of a walking stick, and vibration reducing shims.