Try R&E cycles in Seattle. Your problem is common for tandem folks who want campy ergo shifters but who need the cassette range of a shimano mountain set (12/34). My understanding is that there are now 3 campy der. cage sizes and they use the largest in the back with a shimano front der, campy ergo shifting, and a shimmano mountain cassette. They use this specificly for the problem you are describing and are very successful with it. Another solution less expensive - which is not perfect, but gets at your problem. A number of years ago campy built a lower chain ring (24 and 26) for their mountain groupo's, this smaller chain ring is the same bolt pattern as the campy racing triple 9 speed. So I now have two bikes with campy ergo shifting, long cage campy rear der., running a campy triple (50/40/26 or 24) with the new campy 9 speed 12/28 in the back. This is a medium fix in my opinion, but much better than a 50/40/30 with a 12/26 in the back. These small rings come up for sale on ebay fairly often and I stashed away several some years back just for the situasion you are describing.