im gonna drop an important tip for new people. Get the treasure finder asap. Equip it. Then once you get the lantern from the beach (i think in a silver chest), grab the light stone from one of the shops (Either pawn shop or magic shop). Then fuse them together to get the Lamp. Equip the lamp, then go into the cavern and keep fighting rock spitters until you get the silver chest with the silver ore. Fuse it with the treasure finder to get the silver varient. Continue farming that stage until you get a gold chest which contains the gold nugget. Fuse it to get the gold treasure finder. Then go to every stage and keep farming it until you get all the silver/gold chests. It'll say in the bottom right corner of the screen how many chests are left. You can see how close you are to getting a silver or gold chest to drop by watching the treasure finder gauge. Once full, it'll turn orange, then red. Once its at red, it'll only take a few more battles to get a chest. The chests have all kinds of stuff to help you progress in the game. There's at least 3 stages that i can confirm that contain bottles. There may be more. But they also contain materials needed to buy spells from elven town, and to craft outfits from the tailer.
I think 1 bottle i found in a stage with the Bees, 2nd i think from the stage with the fairies, and i just now got a bottle from the stage with the stingrays (in the lava area).