What kind of food are you talking about? Pokemon eat all kinds of food. Some Pokémon have been known to eat other Pokémon too. Drinks can be bought from vending machines, and are like soda and lemonade and water. Berries are food too, and Pokémon love them. There is also Pokeblocks and Poffins, made from berries. There is also honey and aprijuice, and most of food items are normal like cookies and stuff. There is also apples and other fruit. There is also a lot of human food, and there is also kibble. There is also parasitic feeding, like the move Dream Eater, which feeds on dreams. Also, there is real life animals in Pokémon diets, like some Pokémon feed on insects.
Where does this all come from? My guess is this.
Other Pokémon: Obviously in the wild
Drinks: Vending machines and manufactured in factories and home made.
Berries: home grown
Pokeblocks, Poffins, and Aprijuice: all mde from berries
Honey: From Flowers or Pokémon with Honey gather
Cookies: Home made
Fruit: home grown
Human Food: where you would normally find it.
Kibble: Stores who make it in factories for home made.
Dreams and other parasitic food: You brain and body
Real life animals: Places where you would normally find it.
Also watch this weird video
Hope this helps!
EDIT: If you are still not satisfied, a lot of things happening are off screen.