How you obtain an instrument to use for a project depends upon whether the instrument has been published or not.
If the instrument is available from a commercial test publisher, you'll need to contact the publisher and purchase the number of copies that you need. (Depending upon the instrument, you may be able to code and analyze them yourself, or you may have to return them to the publisher for analysis.) See the Descriptions of Research Instruments tab for sources of contact information for publishers for particular tests.
If the instrument has only been published in the literature, you need to ask for copyright permission from the copyright holder. Sometimes the journal publisher is the copyright holder; sometimes it is the author of the article. Even when the journal publisher is the copyright holder, contact the people who constructed the instrument anyway because there may be a newer version of the instrument available. (As you can imagine, this sometimes entails contacting multiple individuals between the journal publisher and the authors, especially if the author has moved to a new institution.) See the Research Instruments used in Articles tab for information about finding articles that include copies of instruments used in research.