Shanghai-based Evergreen offers DN-WR521W, a USB mini trackball. The device has a retractable USB cable. The bloack-shaped mouse has four buttons, a trackball of course, then left and right mouse buttons and then a scroll button. It has 800 dpi resolution.
It measures 30(L) x 30(W) x 20(H) mm and weighs just 22g. The price is 1999 Yen.