You just need to go to a dealer as Mr. Shiftright suggested. You don't need proof of ownership though. They will just copy your current key and program it. The key will probably cost around $100 with programming. The remote will be around $100 also.
Side Note: If you had a remote starter put in the car when you bought they have hidden a "spare key" in the dash. You would have needed to do that 2006. You needed to do so the remote start would work. IF.... If that is the case just go to a hardware store and have a key cut for $5. The keey will start the car becasue you have a regular key hidden in the dash.