I would advise you to find a Chihuahua Club in your area. You can go to AKC.org and then go to member clubs and you will find a listing of Chi Clubs around the country. I'd find the closest one and talk to the people there. Get a list of reputable show breeders (or start attending shows) and talk to the people there.
To be honest... show "lines" may or may not produce show quality puppies. Dont' get me wrong, they may be lovely puppies and very representative of the breed, but still not close enough to the standard to earn their championship. Also, you can tell "some" what a puppy is going to look like as an adult at 6-8 weeks but it is a gamble. If you REALLY want to show, then you should buy an older puppy that you are going to be able to tell if it's going to be showable or not. Bites can go off, fronts and rears can go off, there's so much that can change with a baby puppy. The breeder would just pick the baby puppy they thought had the most potential, but it's really a gamble.
Also, have you shown before? I take it you haven't. In that case, you will probably have to co-own the dog if you want a bitch. The show quality girls are very precious and few and far between. They usually aren't sold to people who don't have any ring experience. Just because it takes so much time/effort/money to produce a good quality girl to show, so they want them to go to a home that is going to really campaign them. Obviously, there would be breeding stipulations in the contract, etc.
It's really quite a lot to get into. But if you are determined you want a Chi to show, then I'd go to the people that show them. Not to a breeder that has show dogs in her line. That would give you the best chance of getting what you want. Just my opinion.
Once you get her, you can find a local all-breed kennel club and get involved with the fun shows and the conformation training classes. Your puppy can be shown at 6 months.