There are Power Berries like in every other Harvest Moon game, but in this one there aren't as many. So far only 6 have been found, and many believe that there are only 6.
1. Go around your farm and cut all of the grass. This may take awhile but eventually you will find a Power Berry.
2. On your farm use your hoe to till up the dirt. This one may also take some time but eventually you will find one.
3. Go fishing at Maple Lake and eventually you should catch a Power Berry.
4. Go to the Goddess in the morning without using any energy that day, throw in an offering and then ask for Energy. She will reward you with a Power Berry because you have not used any that day.
5. When you make it to the Sacred Land in the Treasure Hunt Part 2 Ending, use your hoe on the dirt in that area (near the star, heart, square, etc) and eventually you should till one up.
6. Flak will go missing one day, Nic and Nak will come and ask you to help find him. Use your dog to find him and in reward you will be given a Power Berry.