Almost all of the Japanese archive was captured and held mostly in USA. This material is returned in early '50 to Japan but copy stay in USA. So it is more cheap to start search in the NARA or Library of Congress.
But I am sure that somebody will read your topic and direct contact you. language is great problem so in this way I wish you luck in finding best place and contact for you. From my side I would like to direct you on one place very frequently visited by Japanese researchers:
http://www.plays.jp/bbs/zero22/index.html
Also I hope that you will get in contact with David Aiken, gentlemen from USA with great experience and amazing knowledge in the subject. he has help me much in my search for details about Japanese airplanes so I am sure that he could be helpful for you too.
Cheers