Greetings, 650B tires and tubes seem to becoming easier to find. Here are a few onl ine sources and I'm sure there are several more.
http://www.velo-orange.com/
This past year I've mainly been riding my 650B Rivendell Bleriot ...(and t he old Paramount when it's not raining....August and September!!!)... and w hile not a "classic" it kind of looks like one.
Rob Markwardt
Seattle, WA