These are good places to search for grey literature:

• World Health Organization International Clinical Trials Registry Platform (ICTRP; link opens in a new window): a searchable registry of clinical trials from all around the world
• Sources of grey literature (link opens in new window) focused on research and policy documents from countries in the Global South
• Overton (link opens in new window; University of Cambridge login required): a searchable index of policy documents, guidelines, think tank publications and working papers
• ScanMedicine (link opens in a new window): 11 clinical trials databases from across the world can be searched simultaneously.
• UK Clinical Trials Gateway (link opens in a new window): a searchable database of non-commercial clinical trials
• ClinicalTrials.gov (link opens in a new window): a registry of clinical trials in the US
• Trials Register of Promoting Health Interventions (TRoPHI; link opens in a new window): a searchable database containing details of over 7750 clinical trials, with a focus on public health
• NICE Local Practice Case Studies (link opens in a new window): a collection of over 500 working examples of quality improvement in health and social care services
• EThOS (link opens in a new window): the British Library's database of doctoral theses
• The King's Fund Library  (link opens in a new window): this collection incorporates the National Grey Literature Collection, and covers health and social care policy and management.
• Social Care Online (link opens in a new window): grey literature for social care and social work
• Google Advanced Search (link opens in a new window): search for words appearing on specific websites and/or specific file types — very useful for finding reports, datasets and similar types of material. Lancaster University Library has created tips for filtering Google Advanced Search results to websites from specific countries (link opens in new window)
• It may also be worth using CTRL-F (or Command F on a Mac) to search on specific websites, such as key conferences in your subject area

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 Grey literature includes unpublished theses, conference proceedings, government reports, unpublished trial data, and more.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).

THe NHS Learning Academy have created a guide to Grey literature (link opens in new tab) which explains why and how to use grey literature and some sources of grey literature.