The traditional way to search for journal articles on your topic is to use an index, such as Historical Abstracts, which then provides you with an abstract and the citation you need to locate the journal in your library or request it through interlibrary loan. These days, many indexes contain the full text or provide a system that links you to the full-text in another database such as JSTOR.
JSTOR is a collection of digital backfiles of journals from a range of subjects. It also includes a searching function and since it will let you search into the full-text, many people find JSTOR immensely satisfying. A fairly recent feature includes citations to articles held outside of JSTOR, such as in the Project MUSE collection. So the waters are getting muddy.
My suggestion is that you start your search with Historical Abstracts and/or the Bibliography of Asian Studies. If you can't find what you need turn to searching within JSTOR.
Books on twentieth century China in our library catalog have been cataloged using a range of subject headings depending on the actual focus and time period of the book. For example:
But you may also need to search for:
If you can't tell whether or not a book would be useful to you, try looking inside by using Google Books.