You don't need a mousepad for a trackball.

Correct answer: you don’t.

Trackballs don’t move across the surface, so a pad doesn’t help tracking. The whole point of a mousepad (providing traction for the optical sensor) is pointless when the sensor is inside the ball housing.

If you still want one — for wrist comfort or desk protection — look for something low-friction. Hard plastic or glass pads let the ball roll freely. Cloth pads create drag and get grimy fast. A gel wrist rest is more useful than a traditional cloth pad.

A good trackball works fine on bare wood.