Research involves "identifying a problem area, establishing a plan, collecting data, and evaluating the data" (Nieswiaadomy, 2008, p. 6) and research instruments are simply "the devices used to collect data" (Nieswiadomy, 2008, p. 215). Research instruments include those that measure body processes or reactions such as blood pressure or brain activity but they also includes things such as observation or interview guides, rating scales, questionnaires, and psychological tests. Researchers may create their own instruments but finding appropriate instruments created and tested by others can save time. This subject guide is intended to help find:
Nieswiadomy, R. M. (2008). Measurement and collection of data.In Foundations of nursing research (5th ed., pp. 210-230). Upper Saddle River, NJ: Pearson Education.
It is important to realize that research instruments are copyrighted. Even if an instrument is available full text in a published source, you must obtain permission from the author to use the instrument in a research project unless the source states that the instrument is free for use by researchers. Sometimes a researcher must pay to obtain copies of an instrument for a research project. Mind Garden is an example of a site that sells research instruments to researchers.
"Ideal for health professionals who develop new instruments, adapt existing ones, select instruments for use in clinical trials or in clinical practice, interpret information from measurements and changes in scores, or undertake a systematic review on instruments, Measurement and the Measurement of Change: A Primer for the Health Professions provides a "gentle" introduction to and overview of complex measurement content that helps ensure high-quality science and excellence in clinical practice. While the book assumes a basic understanding of statistics and statistical inference, the statistical content serves to enhance conceptual understanding rather than to guide computations. This highly practical book draws on measurement theory and approaches from a variety of fields"
Measurement and the Measurement of Change by 
orcid.org/0000-0002-3822-526X