You might be able to still find OEM tubes, or very close replicas from places like David Silver Spares in Britain or Yamiya in Japan mate, but they're expensive.
I bought some tubes from Cycle X in the US and they worked fine, and were a lot cheaper than OEM, but I can't remember if there were any differences with internal diameters like you've mentioned?
I'm thinking that maybe you've got some "odd" tubes off a different bike, as pretty much any 35mm fork tubes can be made to fit your forks. Be aware that the '69-72 tubes are totally different to yours, and won't work with your alloy lowers. Cheers, Terry.