Green screen effects and backgrounds abound on the Internet, but Royalty Free doesn't mean Free.  If you are a student or amateur learning to create movies and without a big budget to spend seventy-some-odd dollars or more on each video file, you should check out Pexels web site.  Yes, the site does link to places where you can buy footage; however, there are quite a few truly free video files in all sorts of locations and for a variety of situations.  The free videos and animations are clean, visually appealing, and require no attribution.