Might want to contact them to see if they'll custom install ie. Japanese XP/Vista... Not cheap though, but they have fairly new technology not available to the US market.
On line you can get a Japanese keyboard that you can plug into your laptop USB port as long as you have software that can support it. Perhaps you can go on line to ebay in Japan and look for one.
You can type Japanese with a regular keyboard on english windows.
Just install the language pack
on XP, its in Control Panel > Regional and Language Options > Language Tab > Install files for East Asian Languages
You may need to go to the Details as well
I took Japanese class in college and used this to type up the homework