Grey literature is research and materials produced by individuals or organizations outside of the traditional academic or commercial publishing and distribution channels. Examples of grey literature include:
There are many ways to find grey literature, but sometimes the best way is to go to the source: the websites of nonprofits/NGOs, government agencies, or professional organizations working in education. There are a few databases that one can search for grey literature as well.
Numerous organizations conduct educational research and issue a variety of reports, publications, advocacy briefs, fact sheets and more. These websites can typically be navigated by topic, though you may also look for a Resources or Publications page as well. The Associations page lists several member-based organizations that publish these types of reports as well. However, some may only be accessible to the membership.
This is by no means a comprehensive list. It is a sample of different types organizations supporting research and advocacy in the field of education.
Preprints can be found in a few different places, such as the author's institutional repository or in a more specialized archive. These two archives may be useful for finding education-related preprints.
UCI's School of Education has numerous centers and research labs focused on various aspects of education. See their website for a complete list.