Most established testing instruments require a license (and usually payment to go along with it), but these databases contain many instruments you can instantly download at no cost to you.
Test + Measures Databases
The following databases mostly contain descriptions of tests available for purchase.
Finding Tests in the Databases
Many tests are not formally published but instead simply appear in articles that publish the results of the research for which they were used. Many databases have features that can help you find them.
If you know the specific test you'd like to use...
Finding information about a specific test or instrument in any database (including Google Scholar):
If you would like to explore tests and measures by topic...
Some tests may require permission from the copyright holder before they can be used for research purposes. The copyright holder may be the author of the test or it may be a publishing company. The link below is an excellent guide to copyright/permissions for tests/measures, and includes a template to use when asking an author for permission to use their test.
Copyright: Survey & Instrument Permissions (Edmon Low Library at OSU)