You stop getting it from enemies after a while.
You can get it from COOP, by putting down your whit soapstone, getting invited to another world, and beating the boss. You can also find usable forms as pickups.
Or you can farm it.
Find the gold serpent ring in sens and head back to the sewers. You can plot of circle route there from the bonfire and wearing the ring kill most of the rats and frogs. You'll get quite a few humanity drops every time. Once you have 30, use them as 30 in the upper left and the ring gives u the maximum drop rate (410). now farming you'll get ~5-8 humanity and ~2-4 eyes of death per farm run.
You can also farm the aborted skeletons in TOG, but it's harder to get to and a tad more time consuming.