Online archives of universities, colleges, funding agencies, and other institutions which collect, preserve, and provide free, unrestricted online access to all types of institutional research outputs.
Grey literature is work that falls outside of the ordinary publication stream (reports, proceedings, dissertations, preprints).
Social Media and Email
Sites like ResearchGate, Figshare and others allow researchers to share papers and data outside of traditional channels. Most require registration. You can also contact authors and researchers directly -- most are very willing to help graduate students with their work and will share that missing correlation matrix or other data upon request.