It seems "the usual suspects" [suppliers] don't have the rubber tips
anymore; maybe John Tuttle does ( https://www.player-care.com/ ).

As for the leather cups, so far I've never had to replace one. If they
are stiff, some neatsfoot oil rubbed into the leather will loosen it up.
Usually all I have to do is to spread out the cup a little, with my
fingers, and put it back together. Then clean off my hands as the cups
seem to be coated with graphite! John Tuttle may have new filters, too.

Usually what I find is that the pumps have been put back together wrong,
or someone has tightened down the bolt on the spring loaded valve so it
can't open.

Yes, there are variations on the way these are put together, but most of
the "innards" are interchangeable.

David Dewey