The ingredient Bone Shard is collected from dead Shambles.
4 guaranteed samples can be found, in the following places (magenta crosses on map):
It can also be found randomly in Ahjazda's inventory and in some loot chests.
In most cases, Shambles are not guaranteed to carry Bone Shards; each Shambles has a 50% chance of carrying Bone Marrow instead of Bone Shards. All standard varieties of Shambles are equally likely to be carrying either ingredient, including Decrepit Shambles, Replete Shambles, and Complete Shambles. The only exception are a few Shambles that are guaranteed to have Bone Marrow, all of which are detailed in the Bone Marrow article. No Shambles are guaranteed to have Bone Shard.
Shambles are generated by various leveled lists (Undead, Shambles, and Wilderness lists) starting at level 9. On boss lists, Shambles appear starting at level 7. Before level 9, the only source of Bone Shard is harvested ingredients.
At levels 9 and higher, good places to find Shambles (each of which has only a 50% chance of carrying Bone Shard) are (green circles on map):