These tips apply for evaluating video clips you find online.
When searching online, don't just limit your search to one source (e.g., YouTube). You will also want to try video search engines such as Bing Video or Google Video (which will give you a large number of YouTube results but also results from other video sources). You might also want to try searching Google for [subject] and video/film/documentary, which might reveal still other sources.
Evaluating:
With our Films on Demand and Kanopy databases, you can easily gather video materials from educational documentaries that will enhance your presentation. Both databases allow you to create clips from videos that you can then play back at the appropriate time. You must create an account (username and password) within each database to be able to create/save clips.