Overswing 20 drop locations?

So I just achieved level 128 on my Aran today. Having 15 skill points left, and Overswing at 10, I'm saving them up for when I get my OS20. Problem is, I don't have near enough money to buy the book (it costs 6 times what I have at the moment). I saw some people say about getting it from Papulatus, but on the KMS database, it says only Bains, silver slimes, and dragon rider drops it.

Should I hunt for the book, or just level high mastery or something else?