An easy way to find more sources is to use another source!
Simply look at the references in the current source (eg. the footnotes in an article; the bibliography in a book) and find other sources that may be relevant to your research.
Once you found a citation, use Advanced Search on OneSearch@IU to track it down. You can also ask a librarian for help.
Google Scholar has a feature which tells you who cited an source. Find an article on Google Scholar and click "Cited By".