small plastic ring for slider

I'm referring to the small black plastic ring that helped prevent the slider knob from being pushed down to par (where it eventually scrapes the paint on the casing).

I don't have one and I need one. I'm sure there's a simple item or even a household item I could use for this.

Does anyone know of something that would work well for this?

Google "O-Ring", get yourself a real tiny one of those and that should work