So little interest in the hand operated, don' t know of any repair source. You could try an old time clock repair guy, the guts are similar, gear, hub bushing, and coil spring. The spring is ribbon type coiled up inside a cylinder case on top of the wheeled gear. If the wheeled gear teeth are gone, then no sound, that would have to be replaced from a parts horn.
If you have any luck, like me, the guts are just slightly rusted, so good oil or penetrating oil got things going. My coil spring was working, on the down push for good sound. But poor retract to send the hand button back up.
Fixed that with an "additional" coil spring wrapped externally around the stem of the button. That did it, push the button and honk, and the button resets back up!