As the item description says, Wopal Shell can be harvested anywhere on Wopal. I just got a few from Seabed
As @AndrlCh said, we only have a choice to use medium-size boosting food now in NA. Ruins Shrimp Tempura (+100% medium-size) and Underground Shafts Mushroom Mix (+100% medium-size) are the dishes we currently have. Both are just 100% boost so might not have a great effect, unfortunately.
@Miarkraim If you are going to use medium-size booster, maybe you'd better avoid Seabed. Seabed has two other medium-sized harvesting items aside from Wopal Shell, so the chance you get Wopal Shell will be relatively lower. Coast and floating facility both has another 3 star medium-sized items, but that's better than seabed.