I had a Border Collie from one of the most highly recognized breeders in the world... Jack Knox. Awesome dog. If you're getting one as a pup, it's alot easier to train them on sheep, or for starters, ducks actually... because they won't challenge the dog nearly as much because of their strong instinct to stay "herded up". And ducks are just alot slower than cattle. Once you've got the basics worked into him on them, then graduate up to cattle.
Mine was just so hot to work, even from a little pup, I couldn't keep him away... he'd tear stuff up and go crazy tied up, if he knew we were down by the cattle... so I let him get started really young... like maybe 8 weeks or so. Well, he got kicked and stepped on by a cow in a crowded pen in a "tight spot" where he didn't have any good escape route (concrete walls on all 4 sides), cut open his rear foot bad... and that made him more timid IF HE WAS IN A PEN WITH NO MEANS OF ESCAPE... or if the pen was deep in mud............. but on firm footing with enough room, he was a real go-getter. LOVED working with him.
Jack gave me a book (that I THINK he'd written?) about training them, and understanding the breed and their instincts, and how critical it is to understand and work with that natural instinct. Can't remember the name of it now right off... I'm sure I have it here somewhere.
Jack Knox Learning Life's Lessons with Stock Dogs I found this one online... it's not the one that I have, but it's the only one I found written by Knox.
Training Border Collies, Barbara Sykes Of all of the ones I found online, this one looks "the closest" to what I have, but I know this isn't it either. The one I had would have been written before 1980 sometime, that's about when I bought my dog.
There are differences in the various stock dog breeds as to their natural instincts... so you might want to learn about them before you decide what kind of dog you might want. If you don't understand those, you might get frustrated because you might be fighting against him with how YOU want him to work.