Article databases like PsycINFO can be used to locate published journal articles that describe, validate, and employ tests and will sometimes include the full text of the instrument.
Resources below provide additional guides to tests and measures, including many unpublished or non-commercial instruments. Some of these sources reproduce selected instruments.
Directory of unpublished experimental mental measures by Bert A. Goldman, John L. Saunders, David F. Mitchell
Positive psychological assessment: a handbook of models and measures by Matthew W. Gallager, Shane J. Lopez
The clinical measurement package: a field manual by Walter Hudson
Handbook of psychological assessment by Gerald Goldstein, Daniel N. Allen, John DeLuca (Academic Press, 2019)
Handbook of psychological assessment by Gary Groth-Marnat, Jordan A. Wright (Wiley, 2016)
Handbook of psychiatric measures by American Psychiatric Association. Task Force for the Handbook of Psychiatric Measures.
Measures for clinical practice and research: a sourcebook by Kevin Corcoran, Joel Fischer
Guide to assessments that work by John Hunsley, Eric J. Mash