The place where you will find the most silica pearls guaranteed thats not in the ocean is in the river in the northwest part of the map. Warning that this river is for the most part head deep and murky in some parts even on old ark mobile and it cuts through some cold areas so you need fur or scuba pants. If you go to the pond area next to the snow cave you will find loads but spinos or barys can spawn. If you head up north all the way to where the river ends then turn east there's also alot of silica pearls on the shores. On pc and ume silica pearls can't spawn on herbivore island.

However if you dont wanna fly all the way to the corner of the map you might as well tame an angler since 95% of the silica pearls on the map are in waters that are head deep or deeper and can fit an angler, chances are the closest silica pearls to your base are in the ocean, you can ride them without a saddle, they are fast enough to escape anything except tusos, alpha mosas and alpha tusos, and they can harvest 5x more silica pearls by biting them than a human can by picking them up. For example I only get like 20 silica pearls per clam I pick up but my angler gets more than 100 per clam. So just tame an angler any level can work.