"Policy" can refer to anything that establishes requirements, legal or otherwise.
Legal policy generally comes from laws, regulations, and court case decisions. These can be federal, state, or even in the example of regulations, municipal.
Sometimes you will have a specific policy in mind that you want to research (i.e. the Affordable Care Act, or Roe v. Wade). But often, we have just a topic that we are interested in, and need to find a related specific policy. Here are some tools to help discover policies by topic:
For your APA assignment, you will need to locate Technical Reports.
Technical reports may be created by a government agency, a non-profit organization, or an educational institution. Unlike journal articles, they don't go through traditional review channels, so you need to know a lot about the entity that produces them (a Google search can help with this).
In this guide, there are a number of places listed where technical reports can be found, including the Congressional Research Service, the PAIS database (they are listed as Books but look for items that are short in length and published by an organization), and the govinfo site (look at Congressional Committee Materials).
For interpretation, historical background, and implementation effects of a policy, you need to find secondary sources of information. The best analysis usually comes from materials written by scholars, or experts, and published either through a editorial review process, such as books published by academic publishers, or a peer-review process, used by many scholarly journals.
These are sites that can assist in finding the actual policy, which is your primary source: