Hi there! You can get food in a few ways. The cheapest way is to go Games > Luck > Apple Tree. With each of your creatu you can pick one apple per day.
The quickest way to bulk buy food is via the NPC Shops (under the shops tab). Many of the shops sell food - Antonio's pizza, healthy food stand, baked goods to name a few. You can use the slider to buy multiple of an item at once, but make sure you have enough tu on hand. NPC shops restock every 15min starting on the hour.
You can also buy food from user shops by searching an item in the search bar at the top.
Hope that helps, let me know if you have any more questions :)