Rick is dead on about back yard breeders. Most just want to breed their dog once so they can keep a pup or let their kids have the experience. Sure you can find a good dog from a BYB, but always run the risk of uncertainty in regards to its future health. Most all of the health issues in labs are hereditary, so buying a pup from a reputable breeder is important when considering a pup. Make sure the parents have the necessary clearances, and look at the grandparents and great grandparents as well. Get references from clients with past pups.
If you are looking for that "bull" look in a lab, you are wanting an English Lab. English labs typically aren't as well known for their ability in the field as American, but they are a bit underestimated. Many English bred labs are titled in the field as well. I currently have two black English females with a nice hunting background if you're interested.