This mutagen is the rarest in the game, and there is a good chance the player will not find any without extensive farming of loot from monsters. There is a good chance of a drop from wraiths and harpies at the Old quarry in Chapter II, also the Endrega queen in Chapter I.
Arguably the best way to find Madness Mutagens is the infinitely spawning Harpies in front of the Visionary's hut. The Harpies there deal less damage and only have 50 vitality but their loot drops are the same as the regular version, with enough patience players can pick up 5-6 Madness Mutagens even in Dark and Insane mode.
Unlike Greater Mutagens which can be generated via Side Effect, this mutagen appears to be exclusive to monster drops.