I think, as Mark implied, that you will find that Octalink triple bottom brackets (needed for most Shimano 9 speed cassette equipped tandems) are getting as scarce as hen's teeth. I searched the big internet dealers 9 months ago and found nothing but a few Italian threaded models. The tandem dealers had some at that time but a friend shopping the major tandem dealers a couple of months ago came up dry. He was looking for Ultegra but managed to find someone with a few 105's in stock.

My suggestion is to check with a local bike shop to see if there is anything available in their network or check for NOS on eBay. Since Shimano has obviously seen fit to drop its manufacturing of compatable parts, it means that those without replacement parts will be forced to replace entire drivetrains, and that will be expensive.

Thanks Shimano! NOT!