Check if the test/measure is published by searching for it in these resources. If a test is published it is usually available for purchase through the publisher and not in the library collection.
Once you have a test's name, a Google or Google Scholar search can be a good place to check if the test is freely available online. BUT even if you find it for free online, you'll need to check if you need permission to use it if you're planning on doing research based on it.
Unpublished tests, measures, scales and other instruments can be found using the same techniques you would use to research other part of your topic - disserations, review and research articles in databases.