This is an old revision of the document!
Japanese bookstores are expensive and they don't ship overseas, except these three:
Book prices seem to be fixed, so you look into shipping costs and service quality in order to choose where to buy from.
http://www.amazon.co.jp/ and http://www.hmv.co.jp/ have English versions whereas http://www.bk1.jp/ doesn't.
I've bought from http://www.bk1.jp/ and http://www.hmv.co.jp/.
http://www.amazon.co.jp/ has a marketplace where used books are sold. These can be much cheaper than new books, but they only ship within Japan so you'll have to use a forwarding service and do some math to check if what you are saving in the book's price is less than forwarding fees.