Where should you start?
First you need to do your homework and research different breeds so that you and your family can decide which breed is the right fit for your home. Take a look at the AKC's The Right Dog For You.

How do you select a breeder?
You should look for a breeder that is knowledgeable about the breed, takes pride in the mastiffs they produce, and will be with you for the life of your dog. You should be able to meet the parents of you puppy to evaluate their temperament and quality. The breeder should also be able to provide you with health testing information (and show proof of that testing, if you request it).

Let's Be Honest:
It is also important that the breeder is honest about the breed's traits and the responsibilities that go with along with owning a mastiff. As wonderful as they are, mastiffs, just like all breeds, have what some would consider to be "undesirable" traits and can be extremely costly to own. Ask yourself the following questions:

Will you be comfortable owning a dog that will most likely out-weigh you and take up your entire couch?

Does drool bother you?

Can you financially afford a giant breed? Giant breeds are more expensive to feed and their medical bills cost more (many procedures are often based on the weight of your dog).

For further FAQs about mastiffs (including important information about health concerns of the breed), please visit the Mastiff Club of America's FAQ.