Eject Button is an item in the “Held items” category (Items). It costs ₽4000.
In-game descriptions
| Black / White · Black 2 / White 2 | If the holder is hit by an attack, it will switch with another Pokémon in your party. |
|---|---|
| X / Y · Sun / Moon · Ultra Sun / Ultra Moon · Let’s Go, Pikachu! / Let’s Go, Eevee! · Sword / Shield | An item to be held by a Pokémon. If the holder is hit by an attack, it will be switched out of battle. |
Where to find it
Not placed on the atlas maps (so far: Red/Blue and Generation 3).
Sources : PokeAPI (items, item_flavor_text, machines) · atlas · pokeemerald species_info.h