Cactus Fruits are a Foraging item in Stardew Valley that can be looted directly from the ground like the other Foraging items such as Coconuts or Dandelions. Unlike most of these however, the Cactus Fruits can be looted in any season, and it can be found only in the Calico Desert.
This means that you will need to complete the Vault Bundle in order to fix the bus, since this is required in order for you to get to the Calico Desert. Once you are there you should see some Cactus Fruits on the ground shortly.
Like all other Foraging items, they respawn after a little while, so you will have to wait a few days for new ones if you have picked all of them from the ground.