Air Balloon

The Air Balloon is a held item that grants the holder immunity to Ground-type moves until the item is removed, either by being hit with a damaging move or by using an item-consuming move like Knock Off. This can be particularly useful for Pokémon that are weak to Ground attacks, allowing them to switch in safely against opponents that rely on such moves. However, once the Air Balloon is popped, it cannot be reused in the same battle.

You can get this pretty early in the game by beating all of the trainers on Route 31.