Chumbo looks expensive.
For anyone else considering this, there's another thread with instructions for getting a PS/2 touch pad very cheaply. Will require wiring up your own ps/2 cable. And you could use a USB->PS/2 adapter if you needed usb.
I bought a .9"x1.7"x1.7" miniature track ball off ebay. There is a guy selling them 1 at a time. I wanted something quick that would sit in the space next to my hand brake and that fit the bill.