Rivendell No great surprise here as far as getting 650B. The real
surprise is that they now have a retail storefront! See their
website for hours.
Harris Cyclery The shop that Sheldon Brown works at in West Newton, Massachusetts.
Tim Rangitsch's shop in Rapid City, South Dakota. Home of the 650B Pacer Project.
College Park & Mt. Airy Bicycles
Larry Black's shops in the Washington, D.C. area. In particular,
see Chad at the College Park location as he has been of great service
to Ron Lau in his 650B conversions.
Hiroshi has been doing 650B bikes for years, even when they weren't in vogue.
Spoke & Sprocket
My LBS, in Tacoma, Washington. They have a good feel for bikes
that aren't quite "normal" and they understand the notion of a
comfortable bike for non-racing.
Popular shop in Boulder, Colorado. Peter Chisolm is well known on
the rec.bicycle.* lists as a fierce proponent of Campagnolo, but also
as a knowledgeable bicycle person. They helped convert a very
small Trek 400 including working around issues with straddle cable
length for the rear centerpull brake.