Grey - or Gray - Literature refers to reports, studies, etc., distributed outside of traditional commercial & academic publishing. These might include internal reports posted on an organization's web site, government documents, or academic theses.
The databases below may help you identify important organizations, but the most recent documents might not yet be included. Visit the organizational web site for additional documents.
Many organizations are known for a particular political perspective, which you'll want to consider as you evaluate their reports. If you're not sure, check the organization web site's "About" page for a mission or values statement, history, etc. Not all are clear about a particular agenda or political leaning, however. There's no comprehensive, up-to-date directory, but here are a few sites for basic information. Note that some don't agree on an organization's political bias.