Tip: If you run into a paywall (or a site asking for money to read an article) try the Unpaywall browser extension, which looks for alternative access in Open Access repositories.
DOAJ (directory of open access journals)
Unpaywall
Digital Commons Network
HathiTrust
SAGE Open
JSTOR
H-Net (Humanities and Social Sciences Online)
ERIC: Institute of Education Sciences
RIBA Library Catalog