There are many things to consider if that's the case.
First thing to know and understand is the fact. About 90% of the breeders are out for one thing only and that's your money! Most of them talk a good game, but the bottom line is "your money".
Next thing to know and understand is that IMO thanks to the amount of inbreeding done over many decades, this breed is full of about everything you can imagine health wise. Way to many pups are put down before two years, a lot before one.
Which brings up the need for testing of breeding stock. There are a lot of test that a breeder can use in an attempt to breed for healthier mastiffs (testing is a very useful tool for all breeders). If you run across breeders who tell you they don't need to test because they "know their lines" so well there's no need, it's a good time to move on to the next breeder.
Good things to ask a breeder: How many of their pups live a long healthy life? How many die of natural causes, verses decease (bone decease, cancer, organ ect). Not that many, if any will be truthful about those questions for usually good reasons, but great questions just the same.
Next thing to watch out for. Breeders love the phrase "Co Own". Which means nothing more then they get FREE breeding rights with your pup if it lives long enough to breed. if they choose to do so. I like to refer to it As "Rent a Mastiff" from them. The catch is you do the raising, you take care of any vet bills, care and feeding bills, then if the pups strike their fancy they having breeding rights and use them to make more money off what you thought was your dog. And make no mistake about it, you will "not" get a break in the price of the pup! So pretty much no loss, risk, or investment for them. What a deal huh? FOR THEM!
If you get lucky and end up with a decent pup, and decide to show the pup, the breeders love that like a child with some candy. You do the showing, you bare the cost and they reek even more money from the pup! Hog heaven for them.
It's hard to find a breeder, maybe even impossible, that is and will be honest with you up front. I know more breeders then I can count off the top of my head, not many of those that I would recommend. A few of them I do believe try to do the right thing no matter what comes up, but hard to find them when looking.
Never fall for the sales pitch the parents are "Champions". A champion these days belonging to a breeder usually means they bought the best known handler they could find who could usually finish a cocker spaniel in the mastiff ring if enough advertising money is spent.
Ask the breeder of the pup's your looking at, how much of any vet bills do they pay if the pup comes down with any bone, cancer or, genetic decease in general, your pup might end up with. If they hesitate with an answer, run, don't walk away!
IMO a healthy well put together mastiff is a pleasure to own. The chances of finding one with no strings attached by the breeder is a job in itself.
Good Luck in your search, do tons of homework before you comment your money to any breeder. Oh and never let them make you think their doing you a "big favor" by letting you give them a lot of money for one of their pups! That's a big way of breeder thinking today.
Beware that if you keep these suggestions in mind while talking with any breeders, let alone ask about any of them. If they think you might turn into a "BIG" problem down the road from those type questions, they'll probably hunt for a dumber buyer, but in the end you'll probably be better off with a lot less heartbreak and expense.
Bottom line, finding a healthy mastiff that looks like a mastiff is probably going to be one of the hardest jobs you've taken on in some time. It can happen and does some times, but few and far between I'm sad to say.