when & where are you staying here? you could find lots of folding bike shops in Japan.

Tokyo Cyclemode International 2012 (a version of eurobike or interbike in japan) will be on Nov 2-4. There are tons of bikes/ folding bikes exhibiting here. http://www.cyclemode.net/english/

you might also want to check out BD-1 or Bianchi (Japan's Birdy folding bike), though it folds quite bigger than a Brompton but definitely lighter & faster than the latter.