They were a very popular option for PowerBooks back in the day. You could buy a pack of multi-colored ones and swap 'em in and out daily if you wanted.
When I discovered them after the fact, I had to look high and low for a yellow one for a Duo, the only one I've ever seen for a Duo, far more rare than the 100s. (Not my favorite but better than gray).
Sadly, at this late date I suspect finding one in something other than a PowerBook would be rare, most loose ones being thrown out as worthless toys and hardly associated with what in my personal opinion is the best mouse control device in history, the trackball. I love it so much I swapped out the trackball bezel from my Duo 280 with my Duo 2300 which was the first come with a trackpad. I hated upgrading to the G3 because it forced me to finally move to the trackpad.
Many have substituted marbles and ball-bearings that have the same diameter. The Duo was problematic because of it's odd size, but I think the 100 series was a pretty common size. The information is online somewhere or get yourself a pair of cheap calipers. (useful for so many things)