My comment applies to our particular breed. I can not speak to this being a general rule. [Try not to completely trash my response, okay?]
The short answer: (so you can skip everything below) is know the person you're doing business with. Avoid out of state and internet purchasing. Try for reasonable access. Visiting the site. Watching the dogs interact, respond to the breeder. A few visits. You want excellent – very consistent - word of mouth. Check out references! Breeders know their stock well enough to match a buyer with what will suit them best. You may think you want one thing, but after explaining your real situation, a breeder can pick out the right pup. Most won’t let you pick the puppy from the whole litter. Some might give you a choice between two, because they know their dogs best.
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The long answer:
Do start early, it will take time if you really care. Good you're clear about what you want. But to anyone else who is reading this, that is most important. Decide between a “high” or “low” energy dog. Figure out what types of dog (within the breeder's program) are being bred for what traits!
The real secret is finding an (ethically) honest breeder. Then ask about (meaning as in understanding) what their real purpose is. It’s a very expensive "hobby" and those doing it for the right reasons seldom make an assumed profit. One setback sometimes can’t be offset by the sale of a litter. Make sure somebody isn’t just trying to make up expenses.
Before the internet it took me 2 years to research. Did not know anyone on the “inside.” That was a real handicap. But I ended up w/ a large circle of names now, who've been great support over the years. And would be happy to recommend any of them! I started out by looking at rescues, adverts, pet stores, friends & acquaintances. If I liked the look and personality of a dog I met on the street, I’d ask for a reference.
The more I read on the subject (particularly the breed) made me realize I would want/need support after my acquisition. So I eventually turned to Dog Shows. One of ours is a benched show, which provides the opportunity to meet & greet personalities (people as well as dogs!). Through that experience I collected names (within my State) and when one didn’t (seem to) have what I thought I wanted, they referred me to others. I worked my list, unintentionally at first, but it turned out that’s what happened. Because breeders like to talk (okay, gossip!). My list was of established (professional) breeders. People can start with the AKC to find a breeder's list, also regional Breed Clubs and Fancier’s magazines. Look at which kennels are showing what kind of dogs. Mainly for conformation? Or for what purposes? Quality show dogs usually have a great temperament (outgoing, engaging), but you need to confirm the reputation of that breeder for health certifications to avoid minimal faults/defects. The breeder you select should be willing to give you references and (of course) health guarantees.
I discovered that the breeders who were comfortable enough to answer all my questions (to help me learn) were not only worth my time, but were equally interested in finding the right home for their dog. These kinds of breeders (are not just making a sale) but will get to know you. And then know how to find right puppy for your situation/preference. You won't be able to tell just from looking at a litter (without experience). The better breeders won’t place an inappropriate puppy with you (just for the sale) because they don’t want the dog returned. Knowing it’s not fair to the dog, or to the next home. Ethically breeders want their own puppies returned to them, not allowing them to be resold, or ending up in a shelter. They usually support their own breed rescue operation as well. They can make excellent matches from those candidates as well.
I went to Dog Shows to mingle and listen to conversations. So that I could ask intelligent questions. And observe personalities. Breeders/exhibitors usually bring along their own dogs to these events. Breeders know their lines well enough to find a dog that will suit your intentions (agility, obedience, working, etc..). And you may have to wait for a litter or two. But however you narrow down your list of contacts, be sure to talk with the contacts' competition (without it being too obvious). As in … “is it true "such and such" about the “name of the breeder” and what’s your opinion?” How they answer will tell you a lot … about both parties! Good breeders honor fair competitors in the ring, and will champion colleagues. But no breeder can always be perfect so there will almost always be some issues. It’s when the issues become a consistent problem, mentioned in several conversations, that you want to avoid. Honest breeders will also admit their own issues up front, and explain how they’ve overcome a problem. Usually by no longer breeding to that line. You want them to be knowledgeable about traditional faults known in the breed.
Also they shouldn’t have more than a (very) few litters per year. Meaning to be careful they aren’t puppy mills with a false front. Claiming all their dogs are "home raised" along with their little kids, when they actually have a "kennel" out back! The right breeder produces litters with the purpose of truly improving the line, by producing the best representation of what the breed stands for (meaning it’s expectation and performance objectives).
Show Dog breeders’ main purpose is to keep the best (most competitive) representation of their breed (in order to win) for themselves. You will not always be eligible for that “best” dog, so it’s important to understand the virtues of which ones are for sale. Sometimes it’s only a fine line between “best” and “next best.” And doesn't even matter to the pet owner.