"A mysterious fruit that empowers those who eat it. The flavor is like a dream... a powerful personal experience, yet difficult to describe to others."

Increases your maximum energy by 34 points.

Locations

Can be bought for 2000 Tokens at the Stardew Valley Fair.
Found in a treasure chest on floor 100 of The Mines.
Get your spouse to 13/12 hearts and speak to them in your home. They will comment about life on the farm and how they are enjoying it, then give you the Stardrop Fruit as thanks.
Sold by Krobus for 20000g in the Sewers.
Obtained from Statue in the Secret Woods by giving it a Sweet Gem Berry from a Rare Seed.
In a letter from Willy after getting the Master Angler Achievement