Robert Klevens of Prestige Auctions is a walking encyclopedia of Japanese baseball. Speaks the language which has to help immensely. I have acquired dozens of funky items from him over the years. His table at the National is always a must for me. He's also authored a couple of reference books on the subject which might be helpful. He's a super-nice guy to boot. Good luck with your efforts.
Here's a couple of bromides I have plus a few of the aforementioned funky items.
Mayhem on the basepath, 1930s menko
Shonen premium Kaoru Betto
