Bones

• Bones from mobs

• Bones from loot

• Bone meal from other sources

• Bone blocks

All Skeleton variants (Skeletons, Strays, Parched, Bogged and Wither Skeletons) can either drop 0, 1 or 2 bones. 

Desert Temples on average yield 20.5 bones per structure (so you should nearly always have enough for obtain bone block). 

Pufferfish, Salmon, Cod and Tropical Fish all have a 5% chance to drop 1 bone meal (unaffected by looting). 

Bone meal can be obtained from filling a composter (once I have done a page on that I'll link it here). 

You can obtain bone blocks from Bastion generic chests (24.4% for 3-6).  Bone blocks as well from Nether Fossils, normal fossils under swamps and deserts, and in Trial Chambers (Skeleton Trial Spawner and Bogged Trial Spawner).