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:
Think tanks and research centers can be excellent sources for current policy research. See these tools for identifying think tanks in your policy area:
Professional associations for government workers can be great resources on current issues and best practices. See also associations for specific roles/professions, such as finance, HR, CIOs, energy officials, etc. Below are just a few examples: