Heart Containers increase your warrior's Heart (HP) by one. They are usually hidden in Red Chests that can be found usually in Legend Mode by destroying boulders using bombs, or capturing keeps. In Adventure Mode, these are usually given to players as A-Rank rewards.
Link
Once you have the bomb, head to the northwestern portion of the map. There's a boulder near the outpost that has the red chest that contains this heart container. Make sure to clear this keep before capturing the Central Keep and East Field Keep as part of the main mission. Capturing these keeps will close off their northern gates for the entire duration of the battle.
Impa
When you're near the cave's exit to the southeast, there's an outpost south of the Store Room. Beside that outpost is a boulder. Destroy it to find the red chest containing this upgrade.
03: The Sorceress of the Woods
Sheik
Go to the Southeast Tree and follow the short path to the south, leading to a hollowed part of a tree. Destroy the boulder there to get this item.
04: The Sorceress of the Valley
Lana
First, head west of the West Ruins, past the western outpost. The boulder is located near a sandfall.
05A: Land of Myth
Sheik
Her heart container is located in the uphill path southeast of the E. Boulder Keep. Destroy the boulder and loot the red chest inside.
05B: Land of Twilight
Lana
You need the Hook Shot equipment to reach the ledge where this chest is found. The Hook Shot can be acquired during the story scenario "The Shadow King". The ledge is located north of the village where you found Agitha. It's right beside the outpost to the southwest.
05C: Land in the Sky
Link
Found in a red chest under a boulder in the middle-south path, beside the wooden bridge.
06A: The Water Temple
Darunia
Once you've opened the Central Hall to rescue Ruto, destroy the boulder to the east to reveal the red chest containing this container.
06B: The Shadow King
Midna
In the eastern entrance of the enemy base lies a boulder. Destroy it to reveal the red chest containing the Heart Container.
06C: Sealed Ambition
Fi
Under a boulder in a corner, east of the Temple Entrance base.
07: The Sacred Sword
Zelda
At the start of the mission, head to the southwest keep and you'll find a boulder outside. Destroy it to find the red chest containing this Heart Container.
Impa
South of the SE Magic Keep is a winding path. There's a boulder along the way. Destroy it to find the chest container this item.
Link
This is found under the boulder in the corner of the path, southeast of the Main Temple's altar.
10: Ganondorf's Return
Ganondorf
Located under a boulder right beside the North Oasis, to the northwest corner.
Ganondorf
This is inside the Fairy Fountain. You have to defeat Lana in the ruins first. She'll then re-appear in the Fairy Fountain and attempt to escape. Head to the Fountain and find the chest there.
Ganondorf
This is found under a boulder near the edge of the river, in the field between the Abandoned Fort and Hyrule Castle. This boulder is found near the bridge leading Hyrule Castle.
Link
In a chest under the boulder along the pass between the Clifftop Keep and South Desert Keep.
14: Liberation of the Triforce
Zelda
Destroy the boulder in the unoccupied keep to the northernmost part of the map.