You need a legal backup of the base game (usually Pokémon FireRed or Emerald). These files end in .gba .
To play these games, you won't typically download a "ready-to-play" game file. Instead, you use a legal patching process. pokemon mew you rom hack download
This is a famous enhancement hack of FireRed. While not solely about Mew, it is beloved because it allows you to catch every single Generation 1-3 Pokémon (including Mew) in a single playthrough without needing to trade or use cheats. You need a legal backup of the base