Bob,
Try B.T.S. for detail parts:
http://www.btsrr.com/btsdet01.htm
There are S scale detail parts that might work, including a three chime horn:
http://www.btsrr.com/pix/02002.jpg
Also there was an O scale three chime horn kit in brass that was available some time ago and should still be available from Precision Scale Co, INC..
Greg