you have to find the map that shows the way. You can find it after you go to Alida island or whatever, then dig in front of all the trees, one should make an oversized hole, go through it, then read the map above you.
In Link and the legend of Zelda, Phantom Hourglass, you get the hammer in the isle of ruins, or the island of the dead.
it is on eddo's island
At the Goron island dungeon
youtube: legend of zelda phantom hourglass by cesaritox09
you get it in the temple of courage at modlia island
in the ice temple on frozen island
In the temple
boomarange
go to gust island
the fourth
In the temple on Goron Island
the bombuchu bag is on goron island