Since Valve have made it official in the latest Steam Deck stable update to change your boot video, reddit user waylaidwanderer made a website where you can easily upload and download these quick clips.
A nice idea, and certainly makes finding one you want a little easier. Useful too, since you can preview the video before you download to see if you will like them. It also includes dedicated instructions below each video, telling you exactly what you need to do to get it working which is a nice touch.
Here's a look over some of the boot videos available and a reminder on how to do it: