Data collection instruments can be found a variety of places: Books, research articles, dissertations, and databases. Books may contain a collection of instruments. See the examples below:
Essentials of Intellectual Disability Assessment and Identification by 
Measures for Clinical Practice and Research: A Sourcebook by 
Rating Scales in Mental Health by Instruments, or measures, may also be located in the Methods section of research articles. Under the Methods section there is usually a subsection labeled as Measures. This will list the instruments used to collect data for that study. Some instruments may be available for free by contacting the author of the study; other instruments there may be a fee to access and use.
Databases, like the Mental Measurements Yearbook (EBSCOhost), provide information on uses of instruments and how to access them. Many of the instruments listed in MMY are commercially available.