Grey literature includes: conference abstracts, presentations, proceedings; regulatory data; unpublished trial data; government publications; reports (such as white papers, working papers, internal documentation); dissertations/theses; patents; and policies & procedures.
For additional databases, look at the Databases by Subject Page
Kugley, S., Wade, A., Thomas, J., Mahood, Q., Jørgensen, A.-M.K., Hammerstrøm, K. and Sathe, N. (2017), Searching for studies: a guide to information retrieval for Campbell systematic reviews. Campbell Systematic Reviews, 13: 1-73.
Contact a librarian to begin your systematic review project
The participation of a librarian on a research team is recommended or required by Cochrane, Campbell Collaboration, and the National Academies of Sciences, Engineering, and Medicine.