Monster that drop 'dark pilfer' according to basilmarket.com
Mask Fish,
Dark Leatty,
Zombie Lupin,
Platoon Chronos,
Luster Pixie,
Malady
I'ld suggest zombie lupin. Sleepy dungeon 2 is a good place to find them.
However, I personally wouldn't recommend hunting for a particular item. It can take hours on end to get the item you want.
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Diamond guide-Diamond buying guide