KensingtonWorks Button4 & 5 (like old TrackballWorks) : r/Trackballs
Kensington Orbit® Optical Trackball Mouse from Posturite
Kensington Trackball Mouse - Orbit Mobile Wireless Mobile Ergonomic TrackBall Mouse with Touch Scrolling, Ambidextrous Design and Optical Tracking - Certified to Work with Chromebook (K77600WW) : Amazon.co.uk: Computers & Accessories
Kensington Expert Trackball Mouse | Back Care Solutions
Kensington Expert Trackball Mouse (K64325) Black Silver USB Tested Works | eBay
Kensington Trackball Mouse - Orbit Mobile Wireless Mobile Ergonomic TrackBall Mouse with Touch Scrolling, Ambidextrous Design and Optical Tracking - Certified to Work with Chromebook (K77600WW) : Amazon.co.uk: Computers & Accessories