They spawn all over astraeos, and they usually have jellyfish near them. All you need to do is shoot the jellyfish with a crossbow and harvest. Should take a couple of them but its fine. Since the oceans in Astraeos are very shallow (with the archelon) its very easy to get silica pearls by killing trilobites. I got 200 after i killed a couple herds. So tame this thing ASAP!