Grey literature is literature that is not formally published in books or journals. It can take many forms eg conference proceedings, PhD theses, technical reports, clinical trial summaries, official publications, reports from relevant associations/public bodies and information from key websites etc.
By its very nature, grey literature can be difficult to locate and you should always be aware of the variable quality of the material you find. Despite these difficulties, it is recommended that you search grey literature as part of a Systematic Review in order to minimise publication bias.
Below are some sources of grey literature. Please note that this is not an exhaustive list.
Healthcare/Clinical Psychology Sources
You can browse the websites of relevant private organisations, charities, government bodies, professional associations, academic institutions, institutional repositories etc to find additional grey literature.
In addition to searching these sources, you may wish to consider getting in contact with experts in the field you are researching. They will often point you in the direction of unpublished resources that you may not find through other routes.