The foundation courses of the MPA program provide lots of information on theory of Public Administration, and your course texts are a good place to start. You might also try:

• skimming articles related to your topic, paying particular attentions to background, literature reviews, or sections labeled "theoretical framework"
• searching our ebook collections for your broad topic, adding the keywords "theory" or "framework" or "foundation"
• using Google or Google Scholar to try to locate top scholars in your area of interest