These are good places to search for grey literature:
It is likely that your systematic review will need to include a search of grey literature as well as the peer-reviewed journal articles found through database searching. Grey literature includes unpublished theses, conference proceedings, government reports, unpublished trial data, and more. Leaving out grey literature can run the risk of biasing your results, as doctor and epidemiologist Ben Goldacre explains in the video below.
Searching grey literature can be challenging. Most sources of grey literature cannot be searched with complex Boolean searches the way you would search a database. The CADTH tool (link opens in a new window) for searching grey literature can help. There is also a useful journal article with supplementary files giving step-by-step instructions on how to search different sources of information for grey literature: Godin, K., Stapleton, J., Kirkpatrick, S.I. et al. Applying systematic review search methods to the grey literature: a case study examining guidelines for school-based breakfast programs in Canada. Syst Rev 4, 138 (2015) (link opens in new window).