Published tests may require that you purchase access directly from the publisher and some instruments require that you are certified to use them.
Try searching for the name of the test in the resources, below. Not all resources will provide you with the full text but they may be able to give you clues about where to look next.
Sometimes tests and measures can appear in appendix to a dissertation. Use the following resources to search for dissertations.
Many research instruments are proprietary and can only be used if you purchase them from the author or publisher. Be sure to obtain permission from the copyright holder before using a research instrument.