Below are OA publishers, databases, and repositories where you can find open access monographs. This list is by no means exhaustive. If you would like to suggest an addition, please contact TKTK.
Amherst College Press: https://acpress.amherst.edu/
DOAB / Database of Open Access Books: https://www.doabooks.org/
JSTOR Open: https://about.jstor.org/oa-and-free/
Lever Press: https://www.leverpress.org/
OAPEN / Open Access Publishing in European Networks: https://www.oapen.org/
OAPSA / Open Access Scholarly Publishing Association: https://oaspa.org/membership/members/
Open Book Publishers: https://blogs.openbookpublishers.com/covid19-information-and-resources-from-obp/#open-books
TOME / Toward an Open Monograph Ecosystem: https://www.openmonographs.org/
*OA Browser extension?? https://libguides.bc.edu/openaccess/findoa