Valguero has 2 great options for chitin. One is killing the broodmother in its trench in the redwoods. The other is killing crabs in the abberation cave. Use megatheriums for both methods for best results. You will get more chitin from killing multiple crabs than the broodmother itself, but if you head into the cave in the broodmother trench once you defeat it, you will gets tons more. Your character will take damage in this cave, so bring a gas mask to avoid this.