The number one rule to buying any dog is stay away from puppy mills. The typically puppy mill puggle will spend its entire life in a 12" x 12" crate birthing litter after litter, while severe health issues go uncared for.
How do you spot a puppy mill puppy? No one is going to admit to being a puppy mill, but here are a few tricks to seeing past the facade.
Stay away from pet store dogs. Most of them come from puppy mills and, even the ones that don't, often have emotional and physical problems by the time they leave the cramped quarters of the store.
Look for quality, not quantity. Any reputable breeder will not specialize in more than two breeds.
A reputable breeder will also be happy to show you ALL of their dog facilities, including the nursery and kennel. They will also be more than happy to show you the dam and sire (if the sire is owned by them).
Check your local shelters. Many of the dogs at shelters are purebred puppies. A quick and easy search on PetFinder.com will give you the run down.