Start here with these databases for general searching, when you want to your search to include books, articles, dissertations, research papers, working and white papers, conference papers and presentations, and other material. Google Scholar is helpful for looking up known citations and exploring resources stemming from those, but not as good for starting a search. NDL Search provides a search for a wide variety of materials held by the National Diet Library.
How to get access to archives, libraries, and museums in Japan, and how to do research once you're there.
Book catalogs not only allow you to obtain books at Penn, but also provide information about which libraries in Japan hold a book, as well as summaries of content (such as chapters and authors in an edited volume). Use these catalogs to begin your search.
You may want to find dissertations and theses related to your topic, especially if a given scholar has not yet published a book on the topic. Use these databases to find and obtain these and other research papers.