source for a plastic ball

[JoeH]

My apologies if this is the wrong place to put this question. I recently bought the book Tool & Cutter Sharpening by Harold Hall and was studying the chapter 9 on Making a Grinder Rest. He refers to a 25mm plastic ball but I haven't found any information or source for that ball listed anywhere.
On Harold's website he has stopped correspondence. Has anyone made one of these grinding rests or can anyone point me to a source for the plastic ball?
thanks,
Joe

[olderdan]

Search on E bay for plastic machine knobs, 100s of them.