For graveyard maps, you search for other maps the mappers of maps you like have made.
I don't think most ranked maps are overmapped in hardest difficulties, the opposite, actually (though, for most songs, it is hard to make a chart that is not undermapped that represents the feeling of the song and is fun to play, especially at lower keycounts). Maps that are overmapped are more common in popular graveyard maps or loved maps than in ranked maps.
