There are a lot of places not to get a working dog, starting with some of the obvious ones:
Don't buy a dog from a pet store, whether you are getting a pet or working dog. Their dogs do not come from "good local breeders" no matter what the stores claim. Good breeders do not sell their pups in pet stores. Period.
Buy a dog from someone who uses their dogs if you need or want a working dog. Not all Aussies retain good working instinct or the drive to get the job done.
A breeder may have "working lines" but if the actual parents, grandparents, brothers and sisters etc. are not being worked you don't know that the instinct to work is being passed down. Critical pieces of instinct can be lost and you are taking a chance that the pup you get will not be as useful as he could be for your operation. Since you are going to be putting money and time into the pup you might as well give yourself the best chance at success.
Don't be afraid to look at breeders outside your area. Breeders of working Aussies are not that common in some areas of the country so you may need to expand your search. Many breeders can ship a puppy, or a road trip can be a vacation or can sometimes be combined with other travel plans.
If you're looking at a web site and most of the dog photos show them standing next to little signs with people wearing suits and dresses it's a good bet that their first breeding criteria is not for a working dog. There are many more show dog/pet dog breeders out there in Aussieland than there are working dog breeders. Be selective.
If you're looking for a cow dog, make sure the dogs in question work cows. And the same for sheep. Many working Aussies will work both types of stock, but some are specialized more toward one or the other. It helps to discuss with the breeder what types of jobs you will expect the dog to perform and listen to what they say about the strengths/weaknesses of their own dogs. If they don't say, you can ask, and if they know what they are doing they can give specific answers about the working traits of their dogs.
If you're not sure where to look, you can always ask for recommendations. There are several working Aussie groups with helpful members who know the dogs and can recommend breeders and litters.
This is a good place to start:
If you contact a breeder and they don't have any puppies available, ask if they can recommend someone.
This one is on facebook:
This is a practical stock work group:
Another good group with some experienced breeders/trainers as members: