Royalty Free Video Clips "cheap or free?"

Some of my students are making a documentary. We are in need of some news clips.

Where are some websites that we could download free video clips that are royalty free?

I-stock has some great videos but they charge around $100 a video which in the business world is probably fine, but now in ours... 

Or does anyone know what is the legality of us using clips from MTV/CNN/NBC/ABC? 

Thanks