Alma
Harrington
The best website for obtaining Chihuahua breeders is websites like petfinder and puppyfinder. These website deals with top notch quality breeders for practically every doggie breed, you name the breed and you have them. I have purchased around 3 breeds (yellow lab, golden retriever and boxer) from the puppy-finder website and all are really good.
I was, at first, highly skeptical from doing any sort of online business but when I heard about the website puppy-finder, I was so delighted, since the website is easy to navigate and do transactions. Now, since you have mentioned in your question that you need to get Chihuahua for your sis then I would like to tell you that the breed is predisposed to small dogie syndrome, where in if you excessively pampers and praises your pet, then it will try to assume the alpha status in the pack and will shift you, the pet owner, from the pack leader status. So get the breed (Chihuahua) but make sure that you train the Chihuahua in a proper fashion. Good luck.