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Finding psychological tests and scales is a two-part process - identify what tests exist that would be useful to you, and then see what it will take to get the actual "instrument".
Many instruments are proprietary, meaning that you have to pay for them. But there are many that are developed and provided for free by academic scholars. Some of these are actually embedded in journal articles describing their development, while others may be available upon request by just emailing the first or "corresponding" author of the article.
Commercially available (proprietary):
Academically available (perhaps, see explanation below):
Search PsycINFO, using a combination of the search terms (put this on one line of the search screen): SU "test validity" OR ZK "tests & testing"
along with the keywords for the topic of the scale you want to find in the other lines.
This finds articles about tests, and often the article that is about the validity of the test, especially the oldest one about that test, includes either the test itself (possibly as an appendix) or at least contact information for the author of the test.
If the researchers who created the test/scale have not "monetized" it, they are often willing to just send it to you via email upon request. You want to make sure you've checked all of their articles first (including appendices and "supplementary material" the publisher provides on their website for the article) and lay out your own scholarly credentials (e.g. you're a doctoral student working on a dissertation on topic X) in the email asking them for a copy of the scale and scoring system.
Depending on your topic, other databases that may have useful articles reporting on new scales that you might be able to get a copy of include: