Slide button losing its tread

I have a few Sony's - older ones because I prefer reading in LRF format. The problem is that on two of my PRS 350s the little knobs on the slide control have come off making it very awkward to turn it on. I'm certain others have experienced this.
I was able to purchase a superior stylus when that broke and I'm wondering if there is anyway to replace the slide control with something better.