If you want some hints, iirc.... 1. Middle island, explore the small islands to the south. Maybe there's one in the balethunder shipwreck to the east. 2. East Island, the very peak of the castle in Inazuma City, also underground in the cave below it. There's one in the islands to the east or northeast. 3. West Island, in a group of islands to the north or a bit northwest
In addition: West Island: There's one way up in the air that you probably won't show on the map if you're on the ground. To get it, go to the teleport on top of the mountain then glide northeastward towards the shore. ---