One can be found randomly in your field while digging with your hoe.
One can be found by exchanging 900 medals at the horse race for the berry.
You'll be given a Power Berry for winning the Horse Race.
You'll be given a Power Berry for winning the frisbee throwing contest with your dog.
The Harvest Goddess will give you a Power Berry if you give her a gift ten days in a row.
You can find a Power Berry on the 100th floor of the mine next to the hot springs.
You can find a Power Berry on the 18th floor of the Winter mine.
During Winter, walk across the lake and search behind the Winter Mine cave by pressing A. You'll find a Power Berry.
You can buy a Power Berry off the TV Shopping Network once you've bought nearly all of the items they have to offer.
If you fish off the pier with a mythril fishing rod in Winter, you may catch a Power Berry.