Someone has probably found this before, but I figured I'd throw it out there. I was just trolling through the internet(s), and I found that Bungie had a library of cutscenes from all three Halo games. They're pretty good quality (better if you have the bandwidth for the bigger ones). Oh, and the Halo 3 has the variants of the "Arrival" cutscene ending ... not that it changes any of the video part, just what sergeant major says.
Halo Cutscene Library
Halo 2 Cutscene Library
Halo 3 Cutscene Library
Halo Cutscene Library
Halo 2 Cutscene Library
Halo 3 Cutscene Library