How would you find a prosthetist who understands upper limb amputation?

ABC or BOC. Abc and boc are certifying boards for individuals in the prosthetic industry. You may find a qualified prosthetist through their directory.
See below. Most orthotists that I have worked with over the years do have some experience with upper limb prosthetics but admittedly do not do as much with them. Sometimes the best bet is to go the rehabilitation medicine department of a larger medical center. Do to their size they probably would have the most experience.