The safest and most responsible way to buy Kelpie puppies online is to conduct a fair amount of research, there are tons of breeders online, and you need to sift through the good and bad ones.
Once you have found some reputable, ethical, and registered breeders, you should try to meet them in person. This is how you can verify them yourself and get a feel of what they are like, and you will also get the chance to see how the puppies are cared for.
Only go through breeders registered with the proper organisations; this could be the ANKC or any other reputable organisation. The breeder should also follow all of the guidelines laid out by the RSPCA.
It would be best if you did some background checks and looked at various reviews. The more you know about the breeder, the better. It is much better to make an informed decision than to follow what you hear and see at first glance blindly.