We could all use a little help gathering & organizing references!
These sites also offer helpful information on gray literature searching:
Just as we must evaluate peer-reviewed literature, grey literature should also be considered critically. Since editors and peer reviewers may not have appraised or evaluated the information, you should be especially careful about the content you find.
One helpful checklist developed specifically for grey literature by Jess Tyndall at Flinders University: AACODS [PDF]: