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If you are suffering from "keyword search burnout", there are other methods for finding material. These methods should be regularly employed as part of your search process.
Read the bibliographies of the resources you have already found – it will help you find additional leads to materials.
Look for "related publication" links. If you use Google Scholar, in your list of results you will notice that it provides "cited by" and "related articles" links. These can lead you to additional information on your topic.
Search by author. If you know the name of an author who has written extensively in your field, use Summon or Google Scholar to see if you can locate more of his/her works. Both Summon and Google Scholar allow you to search by author name (look for this option in their "Advanced Search" screen).
Search for theses and dissertations on your topic. Theses and dissertations have extensive bibliographies. If you can find one on your topic, it is another source for resources.