"Grey Literature is any literature that has not been published through traditional means. It is often excluded from large databases and other mainstream sources. Leaving grey literature out of a systematic review excludes a major section of the available research. To avoid bias and to ensure that the review is as thorough as possible, always search grey literature" (University of Toronto, 2017, para. 1-2). It includes documents such as:
Certain kinds of grey literature can be found in databases. Others are best found by searching the web. Sometimes it is difficult to ascertain the author or organization responsible for the information, which can make gray literature difficult to cite.(SUNY Empire State College, 2017)
Grey literature is not easy to find, which is why it's "grey"! Expect to have to do quite a bit of searching. That being said, with more and more information being published online, it's become a bit easier.
Government agencies, research institutes, organizations, companies or associations will often have a link to "research" or "publications" on their website. If the body produces a large quantity of publications, then they might even have a "database" or "institutional repository" available for searching.
It is helpful if you know which bodies publish in your area of research interest, but if you don't know where to look, you have a few options: