buy it here (the main website, top right corner), or on Impulse
http://impulsedriven.com/dof
Impulse has is owned by game stop (another big online game retailer), and is like steam, make an account, download the Impulse client (i think its renamed to GameStop now), and buy the game on your acct and it will download it and set it up for play.
no special offer if you don't buy it from the main site though
-Blaz