On my home PC I have a  WIN95 keyboard that has a touchpad located in the
centre, directly under the spacebar. The touchpad just requires movement of
a finger or thumb to move the cursor, and doesn't require shoulder movement
at all. Also, since the pad is located beneath the space bar, it doesn't
require much movement to switch over from typing.

I don't have a supplier or a model number (mine is just listed as a generic
WIN95 keyboard with touchpad), but I have seen these keyboards in many
computer stores, and they are relativley cheap. Maybe one of these would be
the answer.