Seems to be just campaign exploring tbh, a lot of the enemies drop bones as loot. it's definitely annoying to get any decent number though, especially since it's required for most crafting
I had this problem at first too but if you campaign enough bones start to pile up (plus buy the max you can from mole market if it's in stock). The more advanced areas seem to have more of them I think
The ‘Discover: Something’ encounter in the campaign can be found in every area and can drop bones. After a few hours of grinding for WW yesterday I got around 400 bones.