I don't know 'bout anyone else, but I found the drop rate of the Fen Strider's to be terrible. I had to kill around 15-25 Fen Striders to gain my 4 items. I found them mostly around The Lagoon

• To save you alot of pain and time, I found the most Fen Striders surrounding The Lagoon.
  
• I suggest circling The Lagoon until you obtain your quest items, you will also run into plenty of Spore's to gain your other items (4).