Best place is pretty much the only place -- an American Girl store or from their website. Their customer service is one of the best around. Once I ordered a Samantha doll outfit that had a tiny yellow spot on it when we opened it. I called AG to find out the best way to remove the spot. They just sent me a brand new one and said to donate the other one to charity.
Another time I ordered a ballet outfit for the Marisol doll. When it arrived it was terribly wrinkled like it had been smushed up in a ball. Since it was made of delicate material, I wasn't sure if I could iron it, so I called AG and asked them. Again, they just sent me a new outfit!
DD has outgrown all of her AG dolls, but they are stored in their original boxes for her daughter (s) some day.