If you looking to buy one... then you can build one pretty easy.
Oscar spinner has a usb version.
Imperial trackballs have a ps/2 version.
IPac has a usb version.
ie, willing to spend the cash, you can something that will take a VERY small amount of configuration.
For mounting. The fastest easiest way in my mind is going to a hardware store and buying a black counter top. Now just drill and place somewhere.
But all in all....
the best way will be to buy one complete
www.slikstik.com