You're right, there are a few sources you can gather the reagent from. The source you are attempting to use is incorrect as that boss does not drop the reagent to my knowledge.
Try these alternatives:
Gardening - Ultra Dandelions drop the reagent at elder harvests (not commonly though).
Fishing - Fishing Chests only in the Pagoda of Harmony has chances of unveiling the reagent.
Bosses - These are the bosses that currently drop the reagent: