These reference works contain useful overviews of a number of topics in East Asian Foreign Policy. My recommendation is to use these volumes to help your search in two ways.
1. Finding relevant keywords and personages in various countries that can be search terms in Worldwide Political Abstracts and ProQuest Newsstand.
2. Using the bibliographies of individual entries to find relevant books and articles.
These are some of your best sources for articles on international Foreign Policy. Their particular uses are noted beneath the individual databases. Some include journal articles, some primary documents and some newspaper articles.
There are a number of specific Journals that the library subscribes to that may be useful for projects on East Asian Diplomacy.