Ketones are not really present in food at all. They are actually a byproduct of fat breakdown that happens inside liver mitochondria. However, there are foods that promote ketone production on a low-carb diet. These are usually low in carbohydrates and high in fat (think butter).
Your body will also turn stored fat into ketones when you limit your carb intake to below 50 grams per day while also eating fewer calories . The only external source of ketones are exogenous ketone supplements. Your body can use these for energy production just like endogenous ketones.