Using the telescope at the lighthouse, you will see it on a seal island to the far right. (You will need the jet ski to get there.) (see related question)
You can get it by digging underground.
Digging underground
they're not made, you find them by digging underground
Mining and digging Underground.
I would suggest digging in the underground. You need an explorer kit to get to the underground.
As far as is known, there is no banana tree on Big Nate Island.
it is @ the top right hand corner
You can find Revives in some Pokémart and while digging underground.
you can find many everstones when digging for treasure underground
all of the stones are obtainable by digging underground
No. It has absolutely no value to the game.
Frogs have nothing to do with the island quest.