After a fair amount of disorganized poking around, I finally located a selection of vibrators, including some quite similar in design to the Hitachi Magic Wand but not actually that item itself, at the Bic Camera store in Yurakucho! They are (or were) near the big display of "massage chairs" at the top of the up escalator on 2nd or 3rd (maybe 4th?) floor. The ones I bought turned out to be quite satisfactorily satisfying, so to speak. I'm sure they can be found in many other places too...
-Ww