I ran a 50x map sample using only the Divination Scarab of the Cloister to spawn in packs of mobs that drop stacked decks. I used ritual to help duplicate these monsters by spawning them in the ritual, and to get the occasional shop item.

Running these maps, you want to look for packs that spawn inside of a ritual so you can duplicate them. Don't do any rituals until you find one of these cases. It is easy to find if you see stacked decks dropped inside of a ritual circle. The mirage also counts as a second map for rituals so remember to check your shop again!

Atlas: https://poeplanner.com/a/6J-C

PoB: https://pobb.in/ZoocxwenNmrr