You could get Panaracer Col de la vie tires in 590/650a from I's bicycles in Japan or Kogswell. I's bicycles in the shop that is responsible for some of the new 650B designs and being a japanese shop they deal in 650a as well.

There are plenty of 590s in the rest of the world. Unfortunately no one is interested in them, at least not any bicycle personalities.

What bike do you have? A raleigh Sports? Does anyone know what sort of rear brake would work with 650B on the Raleigh Sports? A mafac raid perhaps? I'm in the process of modifying a raleigh sports and haven't figured out what I am going to do with the wheels. It has CR 18 590s on it right now, so either I find obscure japanese tires in that size or convert to 650B because I really love the Panaracer touring tires. But then the 650B CR 18 rims are apparently a real pain the ass, worse than the 590 versions and Harris has stopped selling them.