I've converted two bikes to 650B, and am considering converting a 70's Zebrakenko. Some older Japanese bikes will accept a 42mm wide 650B tire with fenders. most will take a 38mm wide tire. These tires will allow you to ride comfortably over most dirt and gravel roads. There are some threads on this forum, and a google forum, groups.google.com/group/650b , that discuss 650B size tires.
Although rims and tires were scarce a few years ago, they are a lot more common now. Your LBS might not have them, but they are available on line.