I made mine from 20 mm ball bearing balls. It was a fairly quick job. If You have fat fingers, make them a little larger.
Clamped them in a vise and ground them flat with an angle grinder, then finished them on the belt sander and finally lapped
the flats to mirror finish. Grind carefully to keep the balls hardened. My flats are about 12 mm.
I mostly use the flat against the work piece when I need them and protect the moving jaw with a piece of brass.
That way nothing gets dented. Was considering making something to hold the ball to the moving jaw, such as a small magnetic ring mount.
But I have found it easy enough to position the ball where it needs to be for the amount of use I have for it.
Also if using more than one ball, it is important to put some soft deform able aluminum or similar between the moving jaw and the balls.
My vote is on the Al rod method as being most generally useful. I mostly use some soft 4 mm Al gas welding rod that works really well.
Erik