What are the chances of finding this dog?

I would get in touch with either animal control or a rescue organization, or else buy a very large live trap. Think of one of those cages that people use to catch rats without killing them, with the pressure plate. If the dog was scared it may be hard to catch him/her. Even with food it may not be a good idea, as you have no idea what could be wrong with the dog, or what sort of temperament it might have. If there's any way you could get your hands on a large animal trap, I'd say throw a blanket over it and place something tasty inside. If the dog is in the area, it will hopefully go for the food and be caught.

But as finding a large live trap like that might prove rather difficult, I would call a rescue group who is willing to help you look for the dog. Animal control could also help, but it's been my experience that they are not always that reliable.

Another thing you can do is post notifications in your area about having seen the dog. Describe it as best as you can, what condition it was in and where you spotted it. Post on craigslist and notify your local vets and animal control. It may be that someone has lost their dog, and they might appreciate knowing where it was last spotted. They would probably have a better chance of catching it, and I bet they would appreciate you helping them, if you can.

Hopefully the rescue you called has been out there and was able to catch him.