a) Japan
b) Preference for import games (esp. from Japan)
I usually go by Japanese re-sellers on eBay or whoever else lives there and is willing to track down the stuff I'm looking for. Out of all my videogames, 90% are the JP versions.
Sometimes I buy from other sellers here and in Europe, but not very often.
Others have used Yahoo! Japan auctions to obtain them, but an understanding of the language and a seller that's willing to ship internationally are required.
My advice: its always better to ask around and seek out the games you want, rather than wait for the games to appear for sale someday.