Like said above it is best to go with an older pup and it is best to find a breeder that is familiar with her line and has been breeding pups for sometime and shows as well as has Champions as either both or one of the parents and even this is not fool proof as two Ch. parents do not always throw Ch. pups. Problem with finding an older pup is usually even the show breeders expect a deposit on a pup way before they ever get to an age where you would be able to see the conformation. That is where the breeder knowing her lines and being able to give you their best educated guess on how the pup will turn out comes in. If they have been breeding for conformation for sometime then they would have the better eye for their pups. It is also very hard to get a great pup for your first show pup as most will keep them for themselves or put them in a experienced show home. You can however get a pup that has the potential to win and is put together well and who knows you could also score big as it is even hard for the breeders to know what the pup will mature to be
You are on the right track by researching and it would be best for you to go to some shows and look at the Chis in the ring if something strikes your interest then approach the handler/breeder? and find out more information on their lines and hopefully you will find someone that stands behind their pup and would be willing to mentor you. That would be your best bet as a beginner. Just remember not to approach the breeder/handler until after the Chihuahuas are done in the ring
Beware of fancy websites and people that do not show themselves but claim to have show pups. You will find that alot of breeders will talk down about the show ring and say that they do not show because of politics but if you truly want to show then you need to find a breeder that is active in the show world.
Alot of show breeders will expect to co-own the pup with you and this can be good or bad, I have seen it go both ways:-/ I lucked up with my breeder (Great Dane) and her contract isnt as stiff as most and I am co-owning and if I finish him (gain his Championship) or if I neuter him she will sign him over to me and I will own him. I co-owning is not really that bad as long as you can trust the breeder that you co-own with? It really means that he is my pup but if I do something that she disagrees with she has the right to take him from me. Most do this to keep their lines from being ruined. But their are some out there that use this to control things
Make sure you ask lots of questions of the breeder and references are always nice as well as putting their name out there to see what kinda of feedback you get. You also must keep a thick skin about you as I have heard from a Chihuahua breeder/handler/show friend of mine that the Chihuahua show world in the US can get pretty nasty
So dont believe everything you hear from the competitors around you