You can only obtain crop hoppers from trading or purchasing them from /shop hoppers. There are a lot of crop hoppers, such as cactus, sweet berry and pumpkin. Crop hoppers have the ability to pick up any of its crop that is dropped in the same chunk as it and distribute them like a normal hopper. They cost between 15,000 and 20,000 to buy depending on the type of hopper. To see information about them in-game, visit /warp hoppers!