I have a set of Al Payson rosewood castanets (Musser supplied him with odds and ends). One broke. Who could help me out? Maybe Mr. Payson himself?

I doubt if Al Payson is still in the castanet business. Rosewood castanets are available from many sources, including Black Swamp Percussion, Steve Weiss Music, etc.
You probably will not be able to purchase (or match) half of a pair.

WM