When using a free instrument, get permission from the author for its use.
Occasionally, a Google search will turn up the full-text of a free instrument. In many cases though, you need to use one of the following sources:
Psychological Tests for student use, a web page hosted by Ron Okada, Emeritus Professor of Psychology, Faculty of Health, York University, Toronto, Ontario, Canada, M3J 1P3. Questions about this web site should be sent to firstname.lastname@example.org
Access the full text of the tests here: http://www.yorku.ca/rokada/psyctest/
Please read Okata's web page carefully regarding the use of the items listed below.
Commercial tests must purchased from the publisher. In many cases, a Google search for the name of an instrument will result in a hit on the publisher's website. Otherwise, here are sources of publisher information.