Originally posted by Bob The Sailor:

You get the key from defeating the elder. Withered bones are obtained from mining scrap or from chests in crypts.

You did not read my post or so it seems.

You only get them in locked crypts and you can't summon the boss without them to get the key.
3 large swamps with many crypts and there all locked and you can't get any bones without the key and you can't get the key with out the bones.

Many times i have been killed and many monsters have fallen and not one of them dropped a bone and and not found any unlocked crypts.