Hi,
Envato has a filter to search for transparent videos. They download as transparent MOV files. No exrta work to mask out backgrounds. It's SO convenient.
Please add transparent videos to the Adobe Stock library. They're so much easier to worth with for designers without as much video experience. Placing transparent videos over an image immediately adds dimension without doing a ton of extra work.
Adobe stock options like these or these say they're transparent, but when you download them they have a solid background. It's annoying.