Always deal directly with a reputable breeder if you're planning on buying a purebred. The breeder should be a member of the American or Canadian Kennel Club. Frequent and/or multiple ads in newspapers can indicate a basement breeder or broker, or finding ads on multiple internet sites. You can also ask a local veterinarian about a source for puppies, check with clubs or get references.
Visit the kennel to meet the breeder and to see the breeding dogs if you have any doubts (The sire, or male dog, may not be present at the kennel as he may belong to a different breeder.) If anything bothers you about the facilities or the breeder, go elsewhere. Reputable breeders will be willing to answer any questions you have about their operation, and will have a few questions of their own for you.
Puppymillers will not provide the right answers always and will confuse things or will smooth over the answers, drawing you away from the real question. Sales people will try most anything acting as if they know all breeds, the breeder and smooth talking you. If things don't sound or look right, RUN!
Arrange to come back to the kennel when the pups are still nursing. Make sure you see the pup you intend to buy nursing with its mother. That way you'll know the breeder isn't trying to pass off a puppy mill pup as the offspring of their dam (mother dog.)
If you don't care if you get a purebred or not, go to your local Humane Society or SPCA shelter, or rescue and adopt a dog. Thousands of dogs purebreds, crossbreeds and mutts are abandoned each year and are just waiting for someone to give them a good home. One estimate says 25 per cent of dogs in shelters are purebred.
NEVER buy a pup on impulse. Expect to be questioned and provide information to a ethical breeder, failure to have a breeder ask questions is a red flag. This animal is going to be a member of your household for 10 years. Take the time to do your research. Find out about the needs of the bulldog you're interested in, and make sure you can meet those needs.
Check references, see if there a club member, talk with other bulldoggers, ask questions, do all the research and take the time to check it out!