If you know the name of an instrument you want to use, one simple way to get started finding it is with a Google search. You might end up at Wikipedia finding a link to the test and its scoring, e.g., the Yale-Brown Obsessive-Compulsive. If the test if owned by a proprietary entity, that will usually come up in this search also, e.g., the Achenbach Child Behavior Checklist is available for purchase through the Achenbach system of Empirically Based Assessment (ASEBA).
If you cannot locate the instrument in Google, try PsycINFO, the database of the American Psychological Association.
Tests, rating scales, and questionnaires are often an essential element of mental health research and clinical care. Locating specific measures can be a time-consuming endeavor. Many instruments are copyrighted and can only be used if purchased from the copyright owner. Some authors of instruments do not mind sharing them without charge but require that you submit requests to use them and provide a description of how the tool will be used. There are some instruments that are freely available to use and can be located over the Internet or as part of a publication. Make sure you are clear about the status of a particular instrument before using it either in your research or your patient care to make sure you are not infringing on a copyright.
If you do not know what instrument you want to use but are looking for what is available on a certain topic or you are looking for information about the validity and use of a specific test, you can search via electronic databases, Google Scholar, or reference books that provide information on tests.