Yes, all these are getting rare. Is there any reason you must stick with Shimano?
There were some great Suntour/Sanshin hubs, as well as Campy.
Good question. I have gathered a lot of 600 EX parts for my build, so would love to have Shimano hubs as well. Much of what I see out there is Maillard. In fact, I have the original Maillard 6 speed threaded hub that came with my 1985 Panasonic. I'm going to take it apart this weekend for an overhaul and I may just re-use it. Again, though, I just love the older Shimano parts, I can't help it.