To get them all, use /function bone_helmet:give

To get them naturally: Throw 10 bone blocks and a diamond on the floor.
It will automatically have bone protection on it. (A custom enchantment)

To upgrade the bone protection, do as follows:

Level 2: Throw level 1 bone helmet onto the ground and 5 bone blocks

Level 3: Throw level 2 bone helmet onto the ground and wither skull

Bone protection does damage to all mobs with bones in a certain radius

Level 1:
2.5 block radius, 3 hearts, damage to skeleton horses.

Level 2:
4.5 block radius, 3 hearts, damage to skeleton horses.

Level 3:
4.5 block radius, 6 hearts, no damage to skeleton horses.

Enjoy!!!