It's difficult to get the rotten flesh, as is a fairly rare drop when foraging, and there are no Pokemon who regularly drop it. Duskull, who spawns in the area, is a very high level for regular farming, and so is misdreavus

Duskull has had its level reduced and its rotten flesh drop quantity increased. Shuppet used to be the source of rotten flesh, but it no longer spawns out of doors on manmade blocks on the starting layer.

Ariados and Spinarak have had rotten flesh added to their loot tables.

Spider eyes, rotten flesh, and ghast tears can now all independently be used for the quest.