Because "Pokémon Indigo League" is a fan modification (a patch), you often cannot find it as a pre-packaged file on standard ROM sites. You usually have to apply a patch. However, for ease of use, many third-party sites pre-patch them.
What you are actually looking for is one of two things: pokemon indigo league gba rom download for android best
[English Download] Pokémon Indigo/Añil v4.0.2 : r/pokemonanil Because "Pokémon Indigo League" is a fan modification
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