ACA was started by Petland and if you know anything about them, you would not want an ACA puppy, chances are it came from a puppy mill. AKC is really the only reputable registry for Yorkies, too many of the other registries take the breeders word for it. If you want a purebred, please go to a reputable breeder who registers her dogs with the AKC. If you don't really care if the dog is a purebred, you can find many mixes in shelter and rescues. Here's more information about registries.
Registries