There are many sources for grey literature. the most obvious one is the internet. If you do use the internet to find grey literature, then we would advise you use Google advanced search rather than the standard Google search box or Google Scholar. A search strategy needs to be robust, so that it has credibility and can be replicated by others. As a result, Google advanced is the best option out of the Google services as although we don't know how the Google results are generated, at least with Google advanced search there is a framework / structure within the search that doesn't appear in Google Scholar, for instance.
Below are some other sources which will help you to find grey literature.