Good evening Grace,
You pose a very interesting question. From my experience, I would say that the sources student's consult should be both reliable, valid and timely. This is not to say that research conducted 5+ years ago is not reliable, but it could be outdated. Secondly, with students consulting the internet with greater frequency these days, understandably the issue of plagiarism and paraphrasing have really become a concern with educators both in the traditional and online learning environments. An ounce of prevention is worth a pound of cure, as the saying goes.
If we can find ways in which to prevent students from taking the path of least resistance, they will come out the other side more connected to the content and should have a deep understanding of the material. Some suggestions might be: (1) providing students with guidelines on which sites are allowable; (2) at the onset of the class, or project, perhaps providing the students with clear guidelines on what plagiarism and paraphrasing entail and, an instructor guidance they can follow for "when in doubt"; (3) depending on the academic level you teach, and the subject matter, another suggestion might be redirecting them to a free online plagiarism checker to use such as http://www.paperrater.com/. Further, letting them know you utilize online resources to check papers this way will serve as a deterrent.