Certain Pokémon evolve using Evolution Stones (Fire Stone, Water Stone, Thunder Stone, Leaf Stone, Moon Stone, Sun Stone). These can be found in various locations, often in the Celadon Department Store or as hidden items. Other Pokémon evolve through trading with another player (e.g., Kadabra to Alakazam, Machoke to Machamp). If you don't have access to trading, these Pokémon will be difficult to obtain. Consider their evolution methods when planning your team.
