I think local trees are your best source. If you're in part of the world where Shagbark Hickory grows it shouldn't be too hard to find a tree or two growing along a country road or in a park. The trees are easy to identify because of the shaggy bark. Before long the nuts should be dropping to the ground and you can simply pick them up. I would scatter them on the ground when the ground is soft and heel them in by stepping on them. I'd collect many more than the number of trees you want since many will be eaten or otherwise fail to become seedlings. Even after they become seedlings many will not turn into adult trees for various reasons.

If you are looking in a particular area and can't find Shagbark Hickory trees, try another area a few miles away. Even though they are widely distributed Shagbark hickory are quite common in some areas and much rarer in other very similar areas not far away. All plant have spotty distributions.