This ingredient is obtained by killing the queen endregas in the Flotsam forests, in Chapter I. Although it is listed as a crafting component, it is not used to craft any items, however it is needed to cast the spell which unlocks the Ancient manuscript as part of the quest "An Encrypted Manuscript" in Chapter III.
Second Queen endrega's pheromones
Since the Enhanced Edition of Witcher 2 you gain a second opportunity to obtain this item in chapter III during the new quests Lilies and Vipers (Roche's Path) or The Secrets of Loc Muinne (Iorveth's Path).
During the quest Lilies and Vipers, while in the forest section you can kill a third Endrega queen. Once you have reached the shore, on the way back towards Loc Muinne stray to the right. If you head up the path you can find three egg sacs, which look like large rocks that you can open. Attack those to spawn an Endrega queen and four Endrega guards. The queen drops Queen endrega's pheromones. Do this if you accidentally sold/used the pheromones earlier in the game.