Realize that searching for an instrument may take a lot of time. They may be published in a book or article on a particular subject. They may be published and described in a dissertation. They may posted on the Internet and freely available. A specific instrument may be found in multiple publications and have been used for a long time. Or it may be new and only described in a few places. It may only be available by contacting the person who developed it, who may or may not respond to your inquiry in a timely manner.
There are a variety of sources that may used to search for research instruments. They include books, databases, Internet search engines, Web sites, journal articles, and dissertations.
A few key sources and search tips are listed in this guide.
If you plan to obtain an actual copy of the instrument to use in research, you need to be concerned not only with obtaining the instrument, but also obtaining permission to use the instrument. Research instruments are copyrighted. To obtain permission, contact the copyright holder to obtain permission in writing (print or email). Written permission is a record that you obtained permission.
It is a good idea to have them state in wiritng that they are indeed the copyright holder and that they grant you permission to use the instrument. If you wish to publish the actual instrument in your paper, get permission for that, too. You may write about the instrument without obtaining permission. (But remember to cite it!)
If someone posts a published test or instrument without the permission of the copyright holder, they are violating copyright and could be legally liable.