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They might have withdrawn it from BattleNet, but it looks like it is still available at Humble Bundle, I could only see the XBox version on Amazon UK when I checked, and I wouldn't trust buying it from eBay as some of the licenses being sold require you to have a specific graphics card to use the license key.
Actually, I just found it on the Battlenet store too.
Open Battlenet, click on the Warzone tab at the top, pick Shop Call of Duty, and you be taken to the page where you can either download and install Warzone from the first tile or buy MW from the second tile.