Finding Gray Literature is not an easy task, here are some tips:
Conduct advanced searches that will limit your results to a certain domain or publication type. Use Advanced Search in Google
Start your searches using specific terminology, if no luck then use broader terms.
Gray literature is often in the .pdf or .doc format.
Look for agencies, organizations, or advocacy groups related to your topic.
Check out Open Access repositories, such as the ones listed in Finding Tools-Gray Literature.