Academia.edu is a fantastic start, but not everything is posted there in full (or at all). You can also try services such as ResearchGate, Google Scholar, or JStor as well. If that fails, look for the author’s site (or their page of an institution site) online as sometimes they have links there for their articles. If the article is older, sometimes writing to them will also help, as they can sometimes freely send out a PDF for the piece. (Note that if you do not pay for Academia.edu you cannot fully search the site, but you can often get to the thing you need by using the researchers name, or the name of the paper, and "academia.edu" in a google search.)
You can also search the publisher’s website. Sometimes they offer things for free (occasionally even whole books), or at least for a reasonable fee for a chance to read it for 24 hours. If the article is part of an academic book rather than a journal, you might even find it accessible, at least in part, via Google Books. (Hint, if you cannot view all of the pages for something, grab a sentence from the last page you could view and search for it again in a different browser or on a different device. Sometimes that will let you see more of the document.)
Sometimes, just typing in the title of the article and “PDF” will take you to a page that has a download for the item. Be careful here though in making sure that this is a legitimate source as there are sometimes sites, often foreign, that are just looking to hijack your computer with malware or a virus.
If you have access to a university library, they can usually source needed items for you as well (and sometimes that can even happen through your local library, especially using ILL for books that are not journals).
Another option is to try to search for other items by the same author. Often, as part of their body of work on a subject, a researcher will write about the topic from several different directions, or include various parts of the information in short articles. You might even try looking for that person's dissertation or thesis online if it was on the same subject. It is possible that the knowledge you seek is located in more than one place, in more than just that one article that you think you need. It is also quite likely that you will learn something you didn’t even know you wanted to know by looking at the bigger picture.
Also, do not hesitate to network with others who share your interest. Often it is likely that someone else has already pulled that piece and they might be willing to send you a copy. Fellow researchers tend to love to geek out when others share their interests and there is so much more we can learn together!