There was a thread not long ago (aha! Here it is!
) that had a similar theme, and one of the responses had a link to this gadget
that might work out even better than a machine. The only knitting machine I've ever used was a hateful beast that killed my back and pretty much only knows one stitch. I'd recommend avoiding that one if I could remember who built it. But this gizmo you can actually use with your own needles, and it just helps support the work. I'll hasten to add that this is the extent of my knowledge of it, but you might research it as an alternative.
You might also check with an Occupational Therapist and see if there's any adaptive equipment available from a medical supply place.
Hope you find something workable. Good luck!