I've no direct experience with this exact part. However, I've lots of experience looking for parts for relatively esoteric camera accessories.
I have found that sub-assemblies of many items are much harder to source than the complete item.
Given that, I would just buy a complete used unit and thus have a decent set of many spare parts including the one you need currently.
A quick search on Google leads me to conclude there are PB-6 available relatively cheap (< $200).
If you can find one with a bad bellows it should be even less expensive.