Best source is to go to the AKC website (www.akc.org) and find local breeders on their list. You may have to get on a waiting list, but you DON'T want to buy from Newspaper breeders. Folks who truly know the breeds are very selective in breeding and will do tests (depending on the typical maladies a breed tends to suffer from). Yes, you may pay a little more, but a great example is a lady in our church wanted a couple of labs and I recommended the same to her. Instead, she bought two from a newspaper add for $400 total. Now, for $600 or $800 she could have bought from an AKC breeder that gets inspected and does tests. Both dogs ended up with a breed ailment. After $2,000 on vet bills one died and the other is sickly. For that much, she could have bought two from a selective breeder!!
Happens time and time again. Be very careful, particulary with large breeds as they tend to have a shorter lifespan and serious hip and knee issues. My grandfather raised English Bulldogs for 30 years and showed around the country. I grew up in that environment and have heard many sad stories from folks who bought their "dream dog" only to find out something horrible later.
The breed you are looking at is a GOB of fun if you love big dogs. I am trying to talk the wife into a Neaopolitan Mastiff someday (she has to get over the drooling issues).
Later,
Lee