Grey literature refers to any information that is not commercially published, and can include a wide range of material types, such as:
How to Search Grey Literature
Knowing where to search for grey literature can be challenging, and can require creative thinking. There are a variety of suggestions in the following library guide from Simon Fraser University: Grey literature: What it is & how to find it.
A good approach is to be strategic and target where appropriate grey sources might be located. For example, if you know of any government agencies, non-profits, professional associations, research institutes, and other organisations that may be producing content relevant to your topic, you may wish to search their website for any publications and reports. Similarly, you could likely exclude certain types of grey literature because they may not be relevant to your particular question.
Searching for grey literature will likely involve the use of a search engine such as Google, so it will be important for you to critically evaluate sources before including them in your research. You will also likely have to scale your search back to include basic keywords.