The ITAC Mouse-Trak Trackball
DT225 are the most smooth trackball. That is, they can spin.
[see Trackballs That Can Spin]
You can find DT225 Trackball online, going for about $300.
ITAC Mouse-Trak Trackball
DT225 trackball ball diameter is ~57mm
Trackball Ball Sizes and Replacement
If you don't want to spend few hundred dollars,
your best bet is probably
also Best Trackballs 2017
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