I find it very frustrating (but I find this entire game frustrating) that black wax idol is required for so many thing yet i have such hard time trying get it!
Seriously, i don't understand!
I think it would be helpful if Eris give us some when you do her bounties after you've maxed out your rank with her...
Just a thought.
Edit: P.S. I know of all the ways you can get them but they never drop for me!