What's the "B"? I haven't been following much lately, but I thought there was just the PC61 - the black version of the PC60. For all the basic PC60 variants, the new revisions have oval cutouts for the LED's and the old revisions have circular ones. IIRC, the new revisions started shipping at the very end of 2001 if that helps narrow it down. In case you've got some version I'm not familiar with, the really important bit between revisions is the switchwork behind the power/reset buttons. If you've got any doubts, describe what you've got there to the vendor and they should be able to tell you what revision you have.
As for vendors, I haven't really shopped around much, but I doubt you're going to see much difference. And the PCMods people have always struck me as good folks to do business with.