http://www.momosystems.com/ runs a lot of Japanese sites catered to Japanese users. They have an english version of their sponsor page but I lost the email so email and they'll give you an addy to their English Webmaster page.
Also, try
http://xxx.xcity.jp/main.htm
The City is the best of the best. I don't think they offer a partnership, but it wouldn't hurt to email them.
-eru