Install Unpaywall to find self-archived (Green Open Access) articles in repositories worldwide
When you find an article in a database, often an e-mail address of one of the authors is mentioned. Contacting the author is usually a quick way to obtain the full text.
Some authors place a version of their papers on a personal webpage. These papers are easy to find with Google.
Visit university libraries. As a student or university staff member you can borrow books for free at other university libraries, if you go there in person and show your WUR-card. Search WorldCat to find out which library has the required journal, book or database.
WorldCat is also a good source to find free articles (choose the tab Articles).
If these free options don't work, then use the ordering service of the Library. This is a fee-based service.