If you are seeking to do a more comprehensive investigation into your topic you will want to look for additional resources in what is sometimes referred to as "grey literature" (see above) which has been defined as: "That which is produced on all levels of government, academics, business and industry in print and electronic formats, but which is not controlled by commercial publishers."
Grey literature usually resides on the web sites of professional associations, national academies, national funding agencies, conference web sites, and other organizations that put out authoritative, credible information within a given field.
Example: A question looking at veterans and traumatic brain injury and opioids might benefit from looking at/for the following web sites:
These are just examples of resources on a particular topic - this is NOT a comprehensive list. Consult with a librarian below to get more ideas.