This is a very informative article with plenty of good ideas and a FREEBIE motion (or animated) background.
Whether you want to make your own motion backgrounds or simply want the freebie, it is worth checking out RevoStock.com
They also sell stock footage, and have a pretty good selection from what I’ve searched.
I liked the freebie. It’s simple. Just a bunch of glitter, kind of like a snow globe.
RevoStock do have new freebies every now and then, so it’s worth subscribing to their newsletter.