Flea markets, yard sales, craigslist...all of those will probably get you a deal too; might not be a brand new box, but it'll probably come with some games.
If you happen to be in western NY/upstate, there's a chain of retro game stores called Game Craze, and they have a ton of original xboxes, games, accessories and what not in stock. Hell they carry items for every major system back to the Atari 2600, so that's worth a shot.