Dissertations, written by graduate students working on a PhD, are in-depth scholarly works in their own right. Dissertations also offer huge bibliographies that you can mine for even more sources.
Dissertations & Theses--offers most dissertations published in the United States, and some from other countries, written from 1861-present. Many of the dissertations are full-text. If you find a dissertation that is not full-text, you can try to order it through Interlibrary Loan.