Animal Shelter. Adopting an animal from a shelter is a great thing to do; it's much cheaper than buying from a breeder and a good idea if you're not dead set on a certain breed. Don't be surprised when the shelter representative asks you a lot of questions -- understanding your family and household is key to making a good match. A common concern is that a shelter dog could have been mistreated and might be aggressive. But most dogs are in shelters for other reasons: a divorce, a death, or a move -- or the owners just couldn't handle the responsibility. Shelter workers won't give you a dog who is known not to like children.
Breed-Rescue Group. If you're sure you want a particular breed but also want to do a good deed and save some money, try a breed-rescue group. You can find one in your area at petfinder.org. Breed-rescue groups collect abandoned dogs from vets, kennels, owners, and local shelters. The people who run the groups are great resources on specific breeds and you can call them with questions, even if you plan to buy from a breeder.
Breeder. If you don't mind spending the cash and you want a purebred puppy, a breeder is the way to go. The American Kennel Club's Web site, at akc.org, is a good place to start your search. Arrange a visit so your family can meet the breeder, the dog's parents, and the puppies, if there's a litter at the moment. The breeder should show you where the dogs live, the dogs' health clearances, and AKC registration papers. You may have to wait for a puppy.
Pet Shop. Generally, people who are serious about buying a puppy go to a shelter, a breed-rescue group, or a breeder. Pet shops cater to impulsive buyers. They sometimes get their dogs from mass breeding facilities (aka puppy mills), where the dogs may be housed in poor conditions. Those dogs are more likely to have health issues, and the workers are not highly knowledgeable.
The Internet/Classifieds. Buying a puppy directly from one of these sources is risky. The Internet can be a good way to find a breeder, but you should always meet the breeder and see the dog in person. Responsible breeders want to meet the families and are very careful to place their puppies in good homes.