The Open Access Button is a free and legal browser extension for chrome that searches a number of different sources to find an open version of the article you are trying to read. It sources articles using institutional and subject repositories and legal databases like unpaywall. It does not use information from legally dubious sources such as Sci-Hub, academia.edu, or researchgate.com. In the case that an article is unavailable the Open Access Button will ask the author to make a copy available in a repository or other appropriate venue.
from openaccessbutton.org