Are there any truck stops near by or a place where truckers stop?
I used to work near an area where there was a Truck Stop and you would not believe how many lost or abandonded dogs were found around there.
Other then that just check the local paper, craigs list, pet stores and vet clinics. If somebody lost him wants him back there will be postings someplace.
I would be going crazy if I lost / or someone stole my dogs, so I would try to find the owner. Then decide if you want another dog or to help find him a good home.