There are many stock video sites available on the web. We’ve used content from 

https://www.pond5.com/ - use their search filters to find looping videos (look great in scenes), or videos with alpha channels/mattes (that can be easily greenscreened).

https://www.shutterstock.com/ - search for ‘keyed’ to get greenscreen-ready videos.

You will need to compress these videos so they will stream and playback well on your users devices. See our articles on formatting your videos to get tips on how to do this.