When searching for a dataset it is helpful to first think about who might collect the data you are interested in. For example, might it be gathered by a government agency; a nonprofit or nongovernmental organization or agency; other researchers? You can then check the websites of likely data collectors.
Also, you can look for publications, articles or government reports, that cite the data, can you tell where they acquired the dataset?
You can find datasets available on the web by searching Google or Open Data Repositories. Also, the library may have access to the data as part of one of our subscriptions. Finally, you can try emailing a request for the data from the authors or researchers.