Hi Paul,
Try:
http://www.yushu.or.jp/english/ . It is the website of the Japanese Philatelic Society. Perhaps an email to them might help you find several dealers.
Also, the third level(?) of Tokyo Sky Tree has a nice collection of Japanese stamps starting with the first ones. I don't know whether there are stamps from other countries on display there.
Someone there might also help you with finding a stamp dealer.
Good luck,
AndrewG