Brake Boosters getting scarce?

I just picked up a 72 Dart Custom 4-Door, 225/auto with front disks. The brake booster is without a doubt bad.
In trying to find another one, neither Advance, NAPA, Oreilley's, or Autozone can get one.
They all list a 50-6206 from Cardone or comparable from Wagner, but after checking with both, neither had any at the factory.

So where the heck do I get one? I drive the car daily, so I can't yank it off, send it out to be rebuilt, and wait on it.

It was bad enough that the pricing I was seeing was $110-$150, because they will only sell it with the master cylinder, but it seems like I can't get one at all now.

Ideas?