Below is a list of general repositories for grey literature.

Czech Republic

Grey Literature Repositories [e-book] http://repozitar.techlib.cz/record/285/files/idr-285_2.pdf

National Repository of Grey Literature [NUSL]
http://www.nusl.cz/?lang=en&language=en

Conference on Grey Literature and Repositories [proceedings]
http://repozitar.techlib.cz/record/1090/files/idr-1090_1.pdf

Netherlands

Grey Literature in the Netherlands (GLIN) Database [Maastricht University]
https://library.maastrichtuniversity.nl/collections/databases/glin/

United Kingdom

Grey Literature [Information Services, University of Edinburgh]
https://www.ed.ac.uk/information-services/library-museum-gallery/finding-resources/library-databases/databases-subject-a-z/grey-literature

United States

Grey Literature: Reports & Reviews [Texas Christian University] [LIBGUIDE]
http://libguides.tcu.edu/c.php?g=656344&p=4967039

MPHE: Associations and Sources for Grey Literature [University of Chicago] [RESEARCH GUIDE]
https://researchguides.uic.edu/c.php?g=252214&p=1682841