Unfortunately I can't answer that very directly or precisely. The main problem is that people have a lot of different ideas of what a farm collie is. Each person really does have to figure out what he wants in order to start off in the right direction with a hope of finding it. Depending on what one has in mind, there are several possible directions and sources.
The other big problem is that there is no single "farm collie breed" I can point to--several different breeds might be called farm collies and shepherds. Furthermore, even standardized breeds are far more variable than we tend to assume, especially in terms of behavioral tendencies. So while some breeds may be more likely than others, depending on one's needs any of the collie-related breeds or mixes could turn up just the right individual.
The possible sources of your farm collie or shepherd are quite wide. There are the two family lines believed to be remnants of the original North American Farm Collie/Shepherd landrace: Nova Scotia Old Farm Collies, and Old Time Farm Shepherds. The English Shepherd breed is also very close to the original landrace breed--and is still really a landrace itself, despite having a breed standard (or actually several breed standards). Beyond these three possibilities, any of the modern collie/shepherd breeds, or crosses between collie-related breeds, might turn up suitable individuals depending on ones needs as well as the variations that can be found within the breed. It is hard to find the breeders in any modern breed that are bucking fashion trends and selecting for farm collie characteristics, but they are out there. People on the farmcollie email list have dogs from almost every one of the modern collie/shepherd breeds, and some are breeders who are breeding for farm collie/shepherd qualities, so this is one good place to ask about possible leads for any particular breed of interest.
There is also a very real possibility of finding your perfect farm collie through the various rescue organizations. Though many rescue groups try to assess the temperament and personality of the dogs in order to find the right permanent home for each dog, most would not have the resources to screen dogs for what might be called "farm collie potential". But some rescue dogs are fostered in farm homes or homes with other animals where much relevant information might be learned about them.
So generally these are the sources for farm collies. But as you start looking or thinking about the different possible sources, be sure to be thinking even harder about what you really want or need.
However be careful that other aspects of the dog will not interfere with its suitability as a companion in your own situation. For instance, a very territorial dog may be so protective of his own family that he is not accepting of visitors, even children. Some dogs have so much herding/chase/prey instinct that they might tend to chase and nip running children or other pets. No matter how much nurturing potential a dog may have, genetically, pups will usually play rough and can hurt small children and other animals by accident. They need to be carefully supervised with children and other creatures, and gently taught how to be gentle.
Despite that generally valid recommendation, some people have found ideal dogs for their farm from among the modern registered breeds, and related mixes, even from lines that have not lived or worked on a farm for many generations. If you already have a breed preference, you can start looking within that breed, for suitable family lines. Expect more variability in working traits in lines that have not been bred in the farm environment. Remember, choosing a pup from parents whose working traits have been demonstrated involves a lot less risk.
Even so, even if the pup has all the genetic potential to be the working dog you want, once you have the pup you must bring it up right for that potential to be realized.
A note on training. People on farms who have good working dogs--the right dogs for their particular situation--often say that the dog learned to work without any training. This gives the impression that a good dog just happens as long as the genes are right. Not so.
Scrutinize the upbringing of the good farm dog, and you will find a dog that has more training than the average obedience star. These pups start out young doing chores and are constantly being molded in subtle ways as they accompany their owners all day long. And a farm day is looooong. It probably doesn't seem like training to the farmer because he is doing what he always does, everyday, instead of having some particular time set aside to teach particular commands deliberately to the dog. Both the farmer and the environment are constantly molding and shaping the dog as it grows and develops. It is like learning a language by immersion, rather than by sitting in a class room. The point is, the farm dog has a LOT to learn, to be a good farm dog, and this doesn't happen in a vacuum; it happens in the right environment, with the right human influences. The dog does have to have the needed genetic propensities; beyond that, the good farm dog is made, not born.
If you are going to raise a pup to be a farm dog, please read The Farmer's Dog by John Holmes. May it never go out of print.
Last updated 30 January 2001.