There are breeders with ethical practices who produce merles. They don't breed
for it, but it's in their lines and so they have merles one in a while.
My cousin has brother and sister chis from a good breeder. One is a tricolor and the other merle. I absolutely adore them, they have the best temperaments of any chihuahuas I have ever met.
You could go the breeder route, or you could look around on petfinder for an adult merle chi rescue. This might be a good thing to do because chihuahua puppies can be a little tricky as far as keeping their blood sugar up, potty training, etc. until they have grown some. And with an adult you already have an idea what kind of temperament it has.
These are her dogs.
ETA: about the ethics, some chihuahua breeders are extremely against breeding the merles because of the controversy over where the merle came from, and so believe the dogs with merle lines had another breed slipped in somewhere.
But, if you're looking for a nice companion, or even to compete in the AKC ring you can find breeders who produce health tested, conformation titled chihuahuas with nice temperaments. As long as those breeders aren't breeding merle X merle litters, I wouldn't feel guilty about buying a dog from them. It's so rare to find people who health test their chis anyway, I'd be ALL for supporting that.