The intent of an evidence synthesis is to synthesize all available evidence that is applicable to your research question. There is a strong bias in scientific publishing toward publishing studies that show some sort of significant effect. Meanwhile, many studies and trials that show no effect end up going unpublished. But knowing that an intervention had no effect is just as important as knowing that it did have an effect when it comes to making decisions for practice and policy-making.
While not peer-reviewed, grey literature represents a valuable body of information that is critical to consider when synthesizing and evaluating all available evidence.
Ask experts in the field for relevant grey literature sources. If you are an expert, include important grey literature sources, and ask colleagues for their recommendations.
Search for theses and dissertations: There are a number of databases dedicated to theses and dissertations, which you can search using your search terms.
Search clinical trials: There may be clinical trials being conducted that are relevant to your research question, but that haven't been published yet or never were published.
Identify government agencies and international and non-governmental organizations that might publish technical papers and reports on your topic.
Search conference proceedings and newsletters: Identify professional organizations that have and/or conferences at which researchers might be presenting work related to your topic.
To find grey literature you can search:
Langham-Putrow, A., & Riegelman, A. (2019). Discovery and scholarly communication aspects of preprints. College & Research Libraries News, 80(9).