None of these places carry the target interfaces for the older boards on their sites. They might if you call them.
Arcade Adventures lists the Galaxy Smart Target Interface, but that won't work with the 6000 series (Also priced high):
https://www.arcadeadventures.com/galaxy-dart-board-smart-target-pcb/
I think I have some 6000 series Super Spider target interfaces still that I was going to put together upgrade sets with (spider/matrix/cushion/interface/board) and I can check next week for those if interested.