This reagent is dropped by pollen-coated insects and beasts, namely, they are the Pollen-Coated Drone and the Pollen-Coated Swarmer. These mobs can be found in many Battle for Azeroth zones, mainly in Tiragarde Sound, Drustvar, Zuldazar, and Nazmir.
Herbalists can also gather the 
Siren's Pollen from the trees in BfA zones, however, it's still better to kill these monsters since they drop 7-11 items at once. However, if the player would rather choose the non-hostile approach, the fastest farming method is to make circles around eastern Drustvar, in Zuldazar around Dazar'alor and in the central area of the Tiragarde Sound.
Using items such as 
Monel-Hardened Stirrups is recommended as it increases the speed significantly, especially when used along with the 
Enchant Gloves - Zandalari Herbalism or 
Enchant Gloves - Kul Tiran Herbalism, which combined provide maximum efficiency.
The 
Siren's Pollen is an important Alchemy reagent, necessary for crafting of most of the potions and flasks, and as such – in constant demand.