In CINAHL, there are two ways you can search for instruments that have been used in studies.
If you know the name of the instrument, you can type the name in the search box and then select "Instrumentation" from the drop down menu before you search.
If you know the topic of the instrument but don't have a specific name, you can search for words related to your topic. In this case, the word has to appear in the name of the instrument.
In PsycInfo, you can also search by the instrument name or topic by using the Tests & Measures feature in the drop down menu.
If you are trying to find a copy of an instrument, PsycTests sometimes has full versions of questionnaires.
You can also try searching the web to see if there is a free copy of the tool available.
Alternately, you can search the Dissertations and Theses database for the name of the instrument, as many full instruments are included as an Appendix of a dissertation.