Reputable breeders will NOT breed for color. They will breed for breed type, health, temperament, etc. Color is quite a bit down the list of important criteria.
That said, you should have ZERO problem finding a nice fawn pup within driving distance. I'm going to be (hopefully!) having a litter of fawn Mastiffs this spring, however I'm not exactly in your back yard.
Do your research on the health of the dogs behind the dog you're thinking of purchasing. Compare the pups parents to the breed standard (which you can find if you spend a few minutes on Google, or on a variety of websites).
It's been my personal experience that breeders who advertise pups at a particular price on their websites are to be avoided, as are breeders who accept pay-pal, credit cards, or similar.
And, lastly, if you are prepared to get a Mastiff, please either rescue one in need of a new home ... OR ... spend the money to get a well-bred one. Do NOT spend a couple/few hundred dollars on a Mastiff you get out of the local paper or on any of the somewhat sketchy puppy-selling-websites. These dogs (generally) have not earned their AKC Championship, have had any health-testing, etc. Just because a 'famous dog' or two appear in the pedigree of the pup you're going to get doesn't mean that your pup will look anything like that 'famous dog' --- in any given litter, on average MAYBE one or two pups will be worth breeding the next generation, and if the dogs inbetween the 'famous dog' and your pup were out of the ones that weren't really worth breeding for the next generation (due to lack of breed type, temperament, health, etc.), your dog will have a better chance to be a disappointment than a dog out of a litter where the parents were selected for superiority in terms of breed type, health and temperament.
Good luck to you on your search.