Biggest tip of advice i can give you, is get the treasure finder asap. Equip it. Buy the light source thing from one of the shops. I think its the magic shop, i cant remember. Then get the pendant that increases your allure (dont remember where i got it). Wear it when you go to the plaza to get Mordred to talk to you. Then follow him to his place to unlock the fuse ability. He gives you a stone tablet which is for something else unrelated (give to fisherman on docks). Go to the beach (same screen as the docks; which is south of the slimes). Making sure you have the treasure finder equipped, keep battling the jellyfish until you get a silver chest. Which SHOULD be a lantern.
At the bottom of the battle screen, you'll see the treasure chest icon with a meter next to it. Each time you battle an enemy (and win), the bar on the meter will rise a little. Once it reaches 100%, further battles will turn it to light orange, to orange, to red. The closer it is to red, the more likely a silver or gold chest will appear. Once you get the chest, the treasure chest meter will reset. If you escape from battle, it will also reset. If you leave the area, it will also reset.
Anyway. Take the lantern and the light source to Mordred's place and fuse them together to get a lamp. Equip it. You should still have the treasure finder equipped. Go to the cave (same screen as the fairy tree and bee forest), and fight rockspitters until a gold chest and silver chest drops. Silver chest has a silver ore. Gold chest has a gold ore. Take the silver and gold ore to the Fuse shop and fuse the silver to the treasure finder, then fuse your newly acquired silver treasure finder with the gold to get gold treasure finder. Equip it. Then start running every stage you can until you get every silver and gold chest. You'll see how many you have left in the bottom right corner of the battle screen. it'll have a number next to them on how many you have left. The stages where you can find wallet upgrades are generally in the silver chests on stages. I want to say one is in the waterfall area (same screen as elf entrance and marshlands) and the desert area (screen south of fairy forest).
If you get every silver chest from every area, you'll get quite a few empty bottles. i'm up to the snowy area (haven't managed to unlock the battle area yet), and i've already got 7 bottles.