Consider using one of the Google search engines (cautiously) when searching for instruments. Remember, things you find on the web may or may not be accurate, and finding an instrument in this way (or any other) does not mean you automatically have permission to use it.
Google Books may help you determine in which book an instrument may have been published.
Google Scholar may be useful for finding articles in which your instrument is mentioned.
Below are a few examples of web sites offering databases of research instruments. The directories of research instrument web sites above include links to many others.
Looking for more web sites that offer research instruments on specific topics? Try one of the sites below for further options.