Simply put, unpublished tests have not been commercially published. Occasionally, we can find them in journals. The two sources listed below will point you in the direction of tests and measurements published in journals.
Goldman, Bert Arthur and John L. Saunders. Directory of Unpublished Experimental Mental Measures. New York: Human Sciences Press, 1990.
These volumes cite unpublished, experimental tests appearing in journals in psychology, sociology, education and interdisciplinary social science journals. Coverage is limited to US journals from 1970-1997. Entries provide a brief description, and a reference to the journal in which it appeared. Later volumes have a cumulative index. Instruments are categorized by general type and supplemented by author and subject indexes.
Chun, K. T., Cobb, S., and French, J. R. P., Jr. (Eds.) Measures for Psychological Assessment: A Guide to 3,000 Original Sources and Their Applications. Ann Arbor, MI: Institute for Social Research, 1975.
A comprehensive bibliography of the tests and measurements available in 26 psychology and sociology journals published from 1960-1970.
Finding tests and measurements, once you have identified them, can be challenging, as actually finding them becomes a question of if the test is published or unpublished.
Published Tests - published tests are those tests available for purchase from a publisher. Tests are valuable commodities as researchers invest many hours in developing and refining their tests and measures, creating a large body of knowledge about their use (reliability and validity data, for example). As valuable commodities, researchers offer their tests and measures for sale through publishers. You are not likely to find a published test freely available, for example, on the Internet.
Unpublished Tests - unpublished tests, paradoxically, refer to tests that are not available for sale via publishers. That is, unpublished in this context refers to tests which were probably published, just not for sale. Unpublished tests and measures are published in journal articles, books, dissertations - in a variety of places.
Another question to keep in mind - if you find a test or measurement full-text (from a book or journal article), does it include the scoring key? Scoring keys tell you how to evaluate the test or measurement. Tests or measurements without scoring keys are practically worthless.
This page contains some sources for unpublished tests and measures - places you might look to find a test or measure.