: Downloading ROMs for games you do not physically own is generally considered copyright infringement in many jurisdictions. It is important to check local laws regarding "backup copies" and abandoned software (abandonware).
If you want, I can:
For the casual player who just wants to play Pokémon Emerald on their phone, any site will do. But for the preservationist, the speed-runner, and the completionist, is currently the gold standard. wwwmaxromscom best