I agree that responsible breeders are not the problem. That said, there may not be homeless mastiff puppies, but there are plenty of homeless mastiff dogs -- too often original owners abandon them not for behavior problems, but instead simply when they find out how big they get and how much it costs to feed them.
We adopted a Rottweiler we found abandoned along the highway at a popular doggie-dumping spot (tried everything to find her first family, but couldn't). She's the sweetest, calmest, friendliest, most obedient dog we've ever met; she was clearly loved and well-socialized. We assume she just got too big for her original owners to handle (and they were too ignorant to give her to a breed-rescue program). If we hadn't been able to keep her, she would have ended up in a rescue program and would have made some Rottie lover very happy.
Nice find!
But is even part of the solution to make these dogs even more rare as I mentioned in post #85?