I don't know where you would ever find one unless you looked on a
site for antique radio equipment but Babcock R/C used to make one.
My father had a Live-Wire Cruiser with a jackscrew servo on the elevator.
It was not proportional it just moved as long as you held the control until
it reached the end of its travel. It wasn't very fast so you just used it to
initially trim the elevator after takeoff and flew the airplane with rudder only
basically.

tommy s