Just by chance, did you check the trunk for the original floor mats yet. When I picked up my Bonneville from the dealership, the detailer had left them in the trunk. I wondered about it for a bit, but I was more interested in checking out the whole car after I got it home. I found them pretty quick.
But yeah, if they aren't there, try asking the dealership about them (whether they have them someplace). If they say they didn't have them when they got the car, ask how much a set will be. Then check out ebay.