http://www.amazon.com/Dirt-2-Xbox-360/dp/B001UEN67Y/ref=sr_1_1?ie=UTF8&qid=1286359298&sr=8-1
I remember it was cheaper on clearance at Sears, but never was able to find a copy there.
Edit
Looks like it's still available directly from Amazon on the link to the right under 'more buying...