Since Wikipedia is a place free of original research (well, in a perfect world) its content is based on information found elsewhere. Books, journals, newspaper articles and websites are all used to verify information in Wikipedia articles. But where do Wikipedia editors find these sources?
Obviously, a Google search is the first thing most will do, but Wikipedia's Find sources page gives a good overview of other places to look, in addition to how to evaluate sources.
Whether you're new to the game, or you want to freshen up on some of the ways you can find good sources to improve articles, The Wikipedia Library is a solid resource. Editors can request access to high quality journals, archives and other reference works, such as JSTOR and MIT Press Journals, through Partnership Databases.
Can't get your hands on a particular resource you need? Try asking at WikiProject Resource Exchange/Resource Request.