Some great ideas in this thread. Thought I'd add a map and street view ( from Google ) to help find some of the tricky Shinjuku ones.
Street Map. I never go to Yodobashi but noted here for orientation :
Shinjuku Used Camera Ichiba, mentioned in the post above, where I now see from the picture below it is indeed more accurately named "Chuko Camera Ichiba". Second floor, above ABC Mart, no windows and a shop entrance from the side street, makes it one you either know or will likely miss. Don't miss it though. At an estimate, at least a hundred or so Pentax used lenses in stock from Takumars, Zeiss and Ricohs to the recent stuff. There is a "Pentax" labelled section towards the back of the shop, but also a dedicated M42 section towards the front on the same wall. Check both !
And if that one is too easy to find, how about Camera Box ( or Box Camera ... or something, my memory is bad
) ? Basement, entrance here :
You can see Map Camera's sign just around the corner for reference. Lots of stuff. Including plenty of junk to be fair, but well worth a browse.
Paul