There are many other sources for grey literature; the most obvious one is the internet. If you use Google to find grey literature, then we advise you to 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, for instance, Google Scholar.
Below are some other sources which will help you to find grey literature.
A scientific citation indexing service that provides a comprehensive citation search. It gives access to multiple databases that reference cross-disciplinary research, which allows for in-depth exploration of specialised sub-fields within an academic or scientific discipline.