-Chapter 13, the top-left chest on the top ship.
-Recruit Stefan in Ch. 15, then talk to him in the Info
menu between battles, and he'll give you an Occult.
-Chapter 21, Tauroneo has one just recruit him with Ike and the scroll will come with him.
-Chapter 27, the boss in that chapter will drop it.
You probably missed the one on the ship, the one where you get Astrid and Gatrie. I guess ultimately it doesn't matter who the last one goes to, it should be someone you use a lot, and not a healer.
--
If we hit that bullseye, the rest of the dominos will fall like a house of cards. Checkmate. - Zapp Brannigan