Guys, even though Quake/etc have been made open source, that is just for the engine. You still need the game data/level files. I believe you need to have a CD with all the game files on it even if you recompile the engine yourself (or create your own levels). I don't believe you will find a site to download the entire game as that would be a copyright violation. Correct me if I am wrong.
Even with the Linux version you have to have the Quake CD with the game data files on it (which are not Windows/Linux specific). At least that is my experience.