I am wondering the same thing. I find a small movie snippet makes a great intro, or break. http://archive.org is a good place to start. Let me know if you find a good place!
Since I am just handing out copies of my project for free I don't think I have to worry about copyright too much?
Honestly I kind of like the oldschool gritty type sound I get from turning my preamps way up and just recording the sample straight off youtube into the mic. Depends what your going for I suppose...... alot of the time with me I am trying to duplicate the noise of me watching a TV and it being turned off. If I want it clear clear (rare) than I bother with downloading/ripping. Audio Hijack is an app that makes this easy ITB. If you are running hardware it is fairly straightforward.
