It depends on what floor of the cave you're in, make sure to use the radar too. From there, if you get them eventually the guy on 2 island will start selling small mushrooms to you. Oh also big hint, mushrooms are always hidden in rocks, and they are in the middle floor... as in, just go down one ladder, and search around with the radar (best to have it registered and just keep pressing the select button), and eventually you'll find big and small mushrooms.