Evidence gathered to inform policy comes from many sources - legislation, regulations, agency reports, news all influence policy development. Conversely, conducting research on the impact of policy is also another angle of research that many seek to understand. There's no one way or one resource to go to for policy research. If you are feeling lost or would like help identifying more sources or direction, ask for help!
You can also visit SU's Law Library. They have different resources that are only accessible on-site. Our law librarians are also a phenomenal resource to advance your policy research.