Some people have friends in Japan who can buy, and post or bring to them.
There is also PenSachi, who sell pens and inks from Japan to worldwide, the have some limited editions as well.
You can also use reshippers like ZenMarket or White Rabbit Express. I think this is the most expensive option from the three, but you can buy from Japanese sites like Rakuten, and Kingdom Note, or Yahoo Auctions JP with them. White Rabbit also has staff that goes to stores in person, for the really-really limited editions. 