Most journals allow authors to upload copies of their preprints/postprints to a repository. Sherpa Romeo
(https://v2.sherpa.ac.uk/romeo/) is an online database that aggregates and analyses publisher open access policies and provides summaries of publisher copyright and open access archiving policies on a journal-by-journal basis.
Directory of Open Access Journals - DOAJ is a community-curated online directory that indexes and provides access to high-quality, open access, peer-reviewed journals. All DOAJ services are free of charge including being indexed. All data is freely available.
Directory of Open Access Books - DOAB is a community-driven discovery service that indexes and provides access to scholarly, peer-reviewed open access books and helps users to find trusted open access book publishers. All DOAB services are free of charge and all data is freely available.
PubMed Central - PubMed Central® (PMC) is a free full-text archive of biomedical and life sciences journal literature at the U.S. National Institutes of Health's National Library of Medicine (NIH/NLM).
HathiTrust - HathiTrust is a partnership of academic and research institutions, offering a collection of millions of titles digitized from libraries around the world.