Instead of performing searches in databases for key words or subject headings, a search for grey literature should first involve considering what types of organizations are likely to be engaged in the type of research you are investigating. Consider:
Once you have identified a likely organization, you can often locate publications on their web site. If the organization’s website does not have a tab or page dedicated to publications and/or research findings, try using the site’s search function to locate what you are looking for. The strategies you use to locate literature on these sites will likely be broader than the strategies used to locate articles in a database. In systematic reviews, for example, the strategy used in a database like Medline will be very narrow and focused. However, the strategies used to locate gray literature will need to be broader. For example, you may search for "clinical practice guidelines" in a database, but search just for "guidelines" when looking for gray literature.
One useful way to search a domain for gray literature is to use the filetype:pdf search filter in Google since a lot of gray literature is in the form of PDFs. You can also use Google to search within a website like this: