As you find books and articles, be sure to mine their references for sources. By tracing cited works you're drawing on the evidence others have used and may find connections that you would otherwise miss. You will also see patterns emerge: works cited by everyone else are worth a look; authors who write a lot about your topic are worth searching by name, etc. Finally, remember that this is the way most scholars search for sources, so if you also search this way, you'll be searching in a very sophisticated and informed manner.
Search for cited books by title or author in library catalogs; for journal articles, check the Do We Have This Journal by journal name to see if we have an article you want. Several databases also include features telling you how often a work has been cited. You don't need to find a book or article on the computer to request it through interlibrary loan. Simply use the reference you have to fill out a form for either a book or a journal article. Blank ILL forms can be found by clicking on My Account on the Library's homepage. Once you're logged in, click on your name in the upper right corner and select "My ILL Requests." From here you'll be able to create a new form. (For essays in a book, ask for the entire book.)
To search forward in time - to see who has cited a work since it was published - enter your original source in Google Scholar and look for the Cited By link underneath the information about the source.