Currently there is no "Legal" method to do this. Best bet is to either check somewhere like gamestop or ebay or a local game place.
Here is a good place to start... Cost 12.95 for a new copy in plastic on ebay.*free shipping*http://cgi.ebay.com/NEED-SPEED-MOST-WANTED-PC-SEALED-NEW-/190440194768?pt=Video_Games_Games&hash=item2c571ec2d0