You would think that a bone is a fairly common find when you scavenge the remains of a dead boar, but for some reason this is a rare drop. Boar bones are an uncommon drop from boars that you find out in the wild in Horizon Zero Dawn. It was quite a while before I found one of these. You can kill quite a few boars before you eventually find one of these.
In general, you should be doing your best to hunt animals all of the time. Animals drop all sorts of materials that you can use in the crafting process. On top of the always important crafting, you will get fairly good with your bow accuracy when you are spending a lot of time killing the tiny critters. Its fairly easy to hit a big ass tank on a machine when you can headshot a bunny.
If you are looking for boar bones, skin or any other elements from other animals. Keep killing them as often as possible and the item will eventually drop. You can usually tell in advance if the item has dropped based on the color of the drop. Boar bones can be found in green scavenging drops. See the scavenging section of the guide for more on identifying the drops.