When you buy a pet, this is one purchase you definitely do NOT want to get from some huge chain. The animals are in jail and have been raised from birth that way, and well, they kind of go crazy and many do not ever recover.
Monroe Mountain Mastiffs is one example of the very pleasant alternative. This is a family in central Utah who just loves this breed.
And what a breed! These large, gentle dogs are huge and make an incredible impression. If that's what you're looking for, I certainly recommend contacting Troy Young, a local professor, who would be happy to talk with you about the joys of raising a mastiff. His web site has lots of information about them, his family and the current puppies available: http://monroemountainmastiffs.com/index.htm