Some of those you may have seen with the more exotic materials may have been one of the many Limited Editions or Special Projects knives that Buck has offered over the years, and were not ordered from the Custom Shop. There are also many that have been shown here that were done by aftermarket craftsmen.
A lot of times when folks describe their knife as being from the Custom Shop, in reality, it's an LE or Special Projects of one type or the other.
Check the auction sites as well as the Bucks for sale sub-forum here. There are usually a bunch of "customs" offered at any one time, especially at auction.