The room with a bunch of poke balls beware of Electrode though catch one he will be usefull because hes ultra fast.
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the master ball is in team aquas hideout. When u find the room with 4 pokeballs the one on the top left (I think) is the master ball.
no where, thats the only one in the game if you dont trade over game.
you battle them
There is a master ball in the Team Magma's hideout in Pokémon Ruby Sapphire, and Emerald. Look in the Hoenn region for the Team Magma's base of operations.
You get it in The Team Aqua Hideout by Lilycove City.