You might try www.kohscamera.com. Let them know you are looking, and if one comes their way, they might call you back.
Also post a message on the Bronica Yahoo!Group--
On that list, Tony Hilton, author of one of the best guides to the classic Bronicas, announced that he was selling off his Bronica collection back in November of 2008, including quite a few rare items like longer Nikkor teles and such. I don't know how much of it he's sold, but he may have a Z or D in there. You could find out how to get in touch with him through the list.