Chinese Hand-Exercise Iron Ball (Baoding Ball)
When talking a walk, i usually carry these. It makes me looks like a Asian sage.
don't buy those with decorative inlaid brass wire (called Cloisonné) on the ball surface. They look pretty, but if you dropped or chipped the ball, the metal wire may stick out. Get pure marble stone, quartz, or solid steel ball. Also, most of them chime. I prefer them not, especially if you work in the office.
In China, there's lots varieties.
there are lots on amazon. Quartz stone, or semi-precious stone, or solid steel balls.
see also Trackball Ball Sizes and Replacement