It looks like there are many places to buy from, but the only place where the two left-click, right-click buttons appear is @ ASA
"Make your mouse movements more easily and accurately when you use this bright, easy-to-install 3' trackball. The large ball means less fine-motor control is required than a standard trackball. Plus the oversized buttons make mouse clicks simple. Compatible with any Windows or Macintosh computer that has a USB or PS/2 port. Also available in switch-adapted form (#W39681)."