These are:

Anker Nielsen is a member of ISJP - International Society for Japanese Philately with a web-site  http://isjp.org/isjp.html

The Japanese Stamps collector society - JPS (in Japan)

http://www.yushu.or.jp/

For a more general information on Japan, see for instance the Japanese turist organisation: http://www.jnto.go.jp/eng/