I got Keepers from a Doberman rescue. A vet in my area is affiliated with the rescue and saw him at animal control and couldn't leave him. She has a vet tech at her office who has white boxers and she fostered him until we got him. All our paperwork were from the dobe rescue but that was about it. We also have two boxer rescues near us. I also looked at a boxer at the humane society before we decided on Keeps.
We found him on PetFinder.com . I'm sure you can find boxers in lots of places and petfinder is a great jumping off point.