I'd say that you should pay attention to how "dog focused" they are. If they are all about crazy rare colours, tea-cups, showy photos of puppies riding in firetrucks, and posting ads all over the web...be careful.
The breeder that mine came from was the one that I initially though was "out there" --it was after I visited all these other "bad" breeders that I realised she was oone doing the right thing.
You want a breeder who genuinely loves the breed, wants to breed from proven KC registered dogs and know about health and temperment.
google some chihuahua clubs and see what comes up...
visit a show if you can and watch and meet the breeders