Click boxes can be picked up secondhand easily still on eBay etc. The control rod that shifts the gears are available from a Bavarian fella who makes them from scratch. You might find that the rubber on the grip shift element of the combo shifter has perished, but you can pick up NOS SRAM shifters of this style or mountain bike style ones out there too.

If you get fed up keeping a discontinued hub drive going, Sturmey Archer makes a replacement hub that does the same thing so the concept of a three-speed hub with a cassette attached is still valid.