If yours is anything like my Focus, there's two screws on the top of the frame rail on the back of the headlight bezel that need to be removed. From there it should pop right out, but you may have to maneuver it around the fascia a little bit. Disconnect the wires to the bulb and you're set.
For the new housing, try ebay. When I cracked my tail light a few years ago, I got one from ebay for $60 as opposed to the $300 that Ford wanted for one.