There’s actually quite a lot of the supplemental stuff (Tardisodes etc.) just hanging around on YouTube, many of them sorted into helpful playlists, which I’m not sure if I’m allowed to link to.
Also for the newer show, I recommend grabbing the torrents from a user called QxR. They’re good quality and most have a folder of featurettes that includes Confidential and various behind the scenes stuff that I was having a hard time finding. I definitely can’t link directly to that but they should be easy enough to track down.
For the older show, there used to be a BIG torrent (like ~250Gb IIRC) with all the existing old episodes in it, but I don’t remember where that came from right now. However, a lot of the old episodes are on the Internet Archive so that might be a good place to start.