Grey literature are publications from research centers, think tanks, government agencies, advocacy groups, professional associations, or other organizations. It falls outside of the "traditional" publishing channels that we often find in academia. They are often not widely disseminated and aren't often indexed by traditional academic search tools, so can be hard to discover.
Tips for tracking down grey literature:
Professional associations for state and local governments are great resources on current issues and best practices. See also associations for specific roles/professions, such as finance, HR, CIOs, arts administrators, etc.
There are so many! Check the Think Tank Search tool or chat with Emily.