There are three ways to acquire livestock guardian dogs.
Buy a pup
Buy a young adult
Rescue a working adult
Buying a pup is the most accessible and common route and it goes without saying to buy a pup from a reputable breeder.
How do you know if you've found a good breeder? By asking lots of questions and trusting your gut. If at any point it sounds too good to be true, it is.
Ask questions about how and where the pups were raised. Was this a planned breeding or just another litter because the parents are not de-sexed? How often do they have a litter? How and where do the sire and bitch live? Where were the pups born?
Are the adults actively guarding stock? Do the pups go with them? Are the pups living up at the house or with the livestock? Are there children around the dogs? Are there any assurances offered regarding the health of the pups? If you have problems can you call them back for help? Do they have some resources to recommend to help you start?
When you buy a pup bear in mind that you will be doing a couple of years worth of training. LGD's are large breed dogs who do not fully mentally mature until close to two years of age. Buying a pup means you need to fill in those two years with manners, discipline, bonding and setting boundaries. Just buying the pup does not make a livestock guardian dog.
Buying a young adult is a less accessible and therefore less common route. It is also more expensive although the cost must be considered in lieu of the benefits.
Finding young adults for sale is difficult. Most LGD's are sold as pups since few people have the capability to raise them to a young adult age. Plus those dogs who are working well are not being sold because they're simply too valuable.
A young adult with a solid foundation of training is really the best way to buy a dog. You have more assurance that the dog is well on its way to being a guardian dog and it saves the rancher the time and effort of raising the pup himself.
Trouble is there are not many people who are keeping pups as long as a year or more and getting them started on the right foot. Those who do so are real gems.
Rescue a working adult. This is probably the least common way of obtaining livestock guardian dogs. There just aren't that many people who are dedicated to caring for and re-training these large, stubborn and independent minded dogs. But there are a few of them out there.
Yes rescues can come with baggage. But the baggage is likely in good shape after passing through rescue since the people doing the rescue don't want to do it twice. A tremendous amount of time and effort goes into rescue dogs.
The screening process rescues use to assist in fitting the right flock guardian into the right environment will help to prevent you being placed with a dog you're not likely to manage very well and to prevent placing a dog in a situation it will not likely manage well with either. People who do rescue are often pretty thorough in their quest for placing dogs in appropriate homes.
If you happen to live within a reasonable travel area of a rescue then you have a great resource on hand and possibly a supply of some good working dogs as well.