"In general, grey literature publications are non-conventional, fugitive, and sometimes ephemeral publications. They may include, but are not limited to the following types of materials: reports (pre-prints, preliminary progress and advanced reports, technical reports, statistical reports, memoranda, state-of-the art reports, market research reports, etc.), theses, conference proceedings, technical specifications and standards, non-commercial translations, bibliographies, technical and commercial documentation, and official documents not published commercially (primarily government reports and documents) (Alberani, 1990)." --From greylit.org
Where to find it: Consider looking at white papers, policy statements, and more from the websites of government (CDC, HealthIT.gov, etc.) or professional organizations (AMIA, AHIMA, AMI, AOTA, APTA, etc.). Sometimes companies will publish grey literature in the form of white papers or market research.