Grey literature refers to a wide range of information that is not formally published. It is produced by different types of national and international organisations, such as government departments and the NHS. Examples include: policies, briefings, doctoral theses, etc.
Grey literature can be hard to find without a specific title. Try searching on the websites of governments, charities, research institutes and similar websites to find relevant papers. Universities around the world often make theses available online though their websites and institutional repositories.
These online resources will also help you find reports and other types of grey literature: