My cousin, who's a Vet, got a nice pup free a years back. Since it was so nice I've been looking too but they cost a nice bit of money. I saw an adoption site once.
I found a Bull Mastiff breeder in my area, and am waiting for a pup now. mom just gave birth a couple weeks ago so i put a deposit down on a girl. yes they have never been cheap. but well worth it IMO.
Heading back out to the breeders place next weekend to visit with her Mastiffs and see how mom is doing, both mom and dad are both champion blood lines so you can imagine the cost of this pup