You'll be explaining the background of your research problem with an eye to giving a comprehensive review of the discussion by scholars of your research problem up to this point in time
Look for well known studies that have been cited by others
Review the references of each article you find to determine if any of the resources they used might be pertinent to your dissertation as well
Search for each article you use in Google Scholar, then click on "Cited by" to see which articles have been written more recently that cite the article you are looking at to see the most up-to-date research on that particular topic
If you can't find the full text of the document, put in an Interlibrary Loan request and we'll try to get it for you, don't skip it!