OK - I'm going to stick my neck out and don a flameproof suit, because this may upset some folks, when it is meant to educate. No matter which breed, there is only ONE right way to buy a purebred puppy. It is not from a mass marketing website, or a newspaper, or a pet store or any place where all you need to do is plunk down money and get a dog. It takes effort, focused toward the right places. In order to get a pup that is healthy and well socialized, you must find a reputable breeder.
A reputable breeder at the minimum:
Breeds only to better the breed
Does not do so for money, and typically makes no money from breeding
A member of local breed club and adheres to the code of ethics
Usually shows dogs - to get of assessment of conformation
Breeds only healthy, well tempered dogs who conform to the breed standard
Breeds dogs that are not directly related and are free from defects
Knows and tests parents for genetic defects
Knows and tests puppies for defects
Provides buyers with all test results and discloses any problems or issues
Socializes puppies prior to placement
Has a spay/neuter contract
Interviews prospective buyers thoroughly
Matches puppy's personality with buyer's requirements
Will be there for you to answer questions about health, behavior, training
You should insist on this behavior from the breeder of your puppy. Your puppy is a 15 year commitment on your part. You owe it to yourself and the puppy that the way it came into this world was not an "oops" but a well thought out plan. In addition, there may be an adult that the breeder wants to place.
How can you find a reputable breeder?
Contact the AKC and find the local or state breed club
Get in touch with the local club chapter
Contact breeder-members and ask lots of questions
Go to a Specialty show and make connections
Go to an all breed show and visit the "meet the breed" booths
Remember, there may not be a pup available at this moment and you may have to wait for the right one.
Chihuahuas specifically....
http://www.chihuahuaclubofamerica.com/
For Chihuahuas in California:
http://www.chiclubnorcal.com/
http://www.southerncaliforniachihuahuaclub.com/
The Chihuahua can have several congenital diseases:
http://www.gopetsamerica.com/dog-health/chihuahua-health-problems.aspx
Best wishes to anyone looking for a purebred dog. I am glad someone took me under their wing when looking for our first purebred dog. And, not all rescues have emotional baggage, we have a great rescue living with us.