If you must have a purebred puppy, go to a responsible breeder or a rescue. If you are looking for a mature dog, some breeders may have them, but rescue groups and shelters are a better bet. In addition to providing lists of available pets, these sites offer a wealth of information on adopting pets and behavior issues. Most breeds have a local or regional rescue group as well as national representation. If you don't have Internet access, look in the phone book or contact the town clerk to find out where the nearest shelter is.
Most shelters have local rescue contacts.
Adopting a dog from a shelter or rescue group is very rewarding, but be selective about a dog. If you want a reasonably mannered dog about two to three years old that gets along with your eight year old son, make sure you get what you want. Don't just fall in love with a dog because he's the saddest or cutest. This dog will be part of your family for many years so it should be appropriate for your lifestyle. Just remember, shelters and rescues will be just as picky about you.