Power Berries are special items that permanently increase your maximum stamina. In Harvest Moon: Tree of Tranquility, each Power Berry increases your maximum stamina by 100 points. There are five total in the game.
Reach the last floor of Ganache Mine(floor 30. Near the east side of the mine, you should see a small, glinting light embedded in the ground. Investigate it, and you will receive the Power Berry. The last level is the spring you find where you get Kingfish.
Go to the Harvest Goddess Spring in Mt. Gelato.
Tip: If you're still trying to get past the momma bear, give it two of any kind of honey a day. After a few days, you will now be able to pass the bear and you will get a short cutscene (You can buy Decent Honey from Brownie Ranch for 135G). The bear also likes shining bamboo shoots and fish. Tip 2: You can only give momma bear 2 items a day. If you give her any other items after you have already given her two, she will get angry.
When your fishing rod reaches level 3, cast it and you will automatically obtain the Power Berry. Best way to get this Power Berry is just to keep fishing. Just don't spend all your time fishing, keep an eye out on your animals and stamina and keep playing the story of the game.
Have an animal with 10 hearts of affection. These must be barn/coop animals, and not pets (sheep, cows, silkworms all work). Brushing animals and taking them outside when it is sunny outside increases their affection. Shear/milk or hand feed them instead of feeding them in their feeding bin.
Till 1000 squares. Best way to get this one is to buy more farmland at the town hall.