I think you need a ditto and another Pokemon then give the other Pokemon a master ball then put ditto and the other Pokemon in the daycare center and then take a walk somewhere away from the daycare center and then go back there should be an egg that egg should be the other Pokemon that you put in the daycare and when it hatches it will be holding a master ball.
WARNING:THIS MIGHT NOT WORK BUT MY FRIEND TOLD ME IT WORKS
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After you defeat Giovanni in Sliph Co. the President will give you the MASTER BALL.
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That is impossible.
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.
Attach your master ball to ONE POKEMON, next CLONE that Pokemon for how many master balls you want. Next detach them, and release all the clones (if you want)