After getting the Whistle in dungeon 2, you can get more heart containers.
Get the Magic Shield
If you don't have 90 rupees, go south three times from dungeon 2, then east four times, then north. Use the whistle to reveal stairs. Go down to get 30 rupees. If you still need rupees, go around killing monsters until you have 90. Then warp to dungeon 1 using the whistle, then go east, south, and west twice. Use the Blue Candle to burn the corner tree, then go down the stairs and buy the magic shield.
Desert Heart Container
There is a desert in the eastern part of the map. Go to the southwest corner of that desert and play the Whistle. Stairs will appear. Go down the stairs and take the piece of heart.
Get the Power Bracelet
If already have the Power Bracelet, then go south from where you got the heart container in the previous paragraph, then go west and push the rock up to make stairs appear. Go down those stairs, then go down the stairs on the right to warp to the mountains. From there, go south, northeast, and east to reach the screen where you got the Power Bracelet.
Otherwise, go to the southwest part of the map. There is a red forest there where the ground is in a + shape. When you reach this area, go north, west, south, west. You will emerge in a new area of the forest. Go north until you are in the graveyard, then go east, north, east twice, northeast, and east. Touch the top-right statue to make it move, then get the Power Bracelet from where it was standing.
Get the Letter
While you are in this area, you should get the letter to the old woman. This will let you buy potions that refill all your hearts. Go north from where you got the Power Bracelet, then go west, then north, then west three times, then south. Push the leftmost rock to make stairs appear, then go down to get the letter.
From where you got the letter, go north, then east. If you want to buy potion now, bomb the north wall a couple of squares to the right of the stairs to open a cave. Go in and equip the letter, then press B to use it. Then you can buy red potion, which refills all your hearts then turns into a blue potion, or you can buy blue potion, which refills all your hearts then turns back into the letter to the old woman.
Get a Heart Container
From there, go east, south, then east until you can't go east anymore, then north, then west. Use the Whistle to reveal stairs. Go down and take the heart container.
Warp to Safety
Go east from this screen and use the whistle to warp to a dungeon.