This page says that an extra Heal staff can be gotten from an enemy in chapter 3. If this is true, it'd probably be Lunatic mode only since I haven't seen a Heal staff on an enemy in Hard mode.
I didn't know enemy loot varied according to difficulty; that's a first o__o
Apparently that Heal staff is indeed Lunatic (and presumably Lunatic+) only.
...geez I could have confirmed that for you. xD
I have so many backup saves from attempts on my first run on Lunatic.