Please dont support a BYB by purchasing a mixed breed dog, find a breeder who does the proper health testing and cares about their dogs. The Mastiff breeder I am looking at does several health tests, and offers a 4 year health guarantee. Let me tell you spending 6000 on surgeries sucks butt.
petfinder is a good way to go. You never know what you are going to get when you purchase a mix from a byb, or someone who is just breeding for top dollar. All breeds have health problems, and although in mastiffs, labs, rotties, boxers, etc. can have an agressive streak.
Consider shelters, and pet finders to find your new furbaby. good luck