From Halopedia, the Halo wiki
The concept art images are usually from the personal blogs of artists who worked on the games, predominantly from Isaac Hannaford's site but also others like Dorje Bellbrook's blog. There are probably more, but I don't have the links handy right now.
Where do you find all the concept art on pages like Anchor 9? If you could tell me or send me a link, it would be greatly appreciated. ~~~~