Numerous 650B tires are available. Here are my favorites:
Michelin's current offering is called Megamium, and is all black. It's a nice tire. The older Axial Raid isn't available any longer, except NOS. These are about 28 mm wide.
Hutchinson makes the World Traveler in various widths, up to about 40 mm. These are great replacements for the old French "demi-ballons." Available in this country from your bike shop (wholesaler is Mel Pinto).
Mitsuboshi used to make/is planning to make a great tire in 32 and 38 mm widths. We used the 38 mm on our PBP tandem.
Michelin, Schwalbe, Vredestein and others also offer some cheaper choices intended for city bikes. Nokian makes a "Rollspeed" that Europeans like, but I haven't seen it over here.
Unfortunately, the old Wolber Super Randonneur and Tandem tires (made from the 1960s through 1980s) are long gone. These were about 32 mm wide, very light, very supple and just wonderful.
Only from hearsay can I report of the old hand-made tires by Barreau, Mutan and others. The riders who knew them tell stories of wonderfully light weight, supple ride and quality. But the price was such that after a special event, the tires came off again, and the standard Michelins or whatever were put back.
As you can see, tires aren't a problem if you are willing to have a spare or two in your basement. Any U.S. shop can get you the tires within a week, but don't expect to enter a shop and find a selection. (But that is equally true for tubulars these days!) That said, I do not know of any current source for true "ballon" tires, which were about 54 mm wide... But even on fire roads, I find the "demi-ballons" (38 mm) usually are enough.
Finally, Harris Cyclery offers some 650B tires, too.
No connection to any of the above makes. I love 650B - glad somebody except Singer and a few French makers are offering that size again. It combines the feel of a "road" bike with the sturdiness and comfort of an mtb wheel. This must be what "the golden mean" was intended to signify!
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Jan Heine, Seattle
Editor/Publisher
Vintage Bicycle Quarterly
http://www.mindspring.com/