Use Journal Articles
Look for peer-reviewed journals using the databases related to your field (i.e., APA PsycInfo, ERIC, etc.). Within these journals, check for articles that discuss the type of measure you are interested in finding. Scan the reference lists of these articles to find the original source of the test or a discussion of the test.
Mine Dissertations
Search the dissertation database Proquest Dissertations & Theses Global. Check the reference list for tests/measures used.
Use the Web
Google the test/measure/scale name, using information gleaned from the peer-reviewed journal articles as your search terms. If you locate an author, request the permission to use the measure. You may find out that rights to the measure could reside with someone else.
Use the Online catalog
Check the Advanced Search page of the online catalog for books containing copies of measures. Type in terms like “test”, "assessment" or "measurement", plus your field (like psychology) and check the record for how that term is used.