After you defeat Giovanni in Sliph Co. the President will give you the MASTER BALL.
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No, you do not need a master ball, if you get it to a low enough hp, you can catch it with an ultra ball. You can find mew in the end of the cave in cerulean.
no
ummm i think you can find it on two island...
go to celadon city and buy coins then go to where you get the prizes, shadow ball is one of them
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.