If anyone's wondering, I found the following page which offers some very good shots for practicing things like rotoscoping and tracking: https://hitfilm.com/forum/discussion/290/free-footage-for-vfx-practice. It's an old post but the download links still work.
The files are MOVs, but you can pass them through an online converter to get them as image sequences. This is the online converter I used: https://www.filezigzag.com/mov-png-en.aspx