This page links to library searches, online databases, and other venues where you can locate a journal article by title, journal, or identifier (such as DOI or PMID). It's a good idea to start with a search engine, as it will have the most comprehensive coverage. Besides, many of the online databases listed below include free full text.
If you're looking for a source that isn't a journal article, try the Find your Source guide. For books, you can also use Special:BookSources to search by ISBN.