I've been searching for an addon that can take videos in game but haven't found any.  Do any exist?  Or do I just have to go to the store and buy video software in order to take in-game videos?

Unless you have a Mac, then it isn't possible. Macs are the only computers that can record video in game as an addon, or the default UI. That said, you could try a third-party program like Fraps or WeGame. You may have to Google those. I am sure there are others, but since I don't record my games, you will have to wait for someone else to answer, sorry.

At least, sadly, I ended your addon quest.