You can trade masterballs from ruby, sapphire and emerald by winning first place in the lottery at the lilycove dept store but you need to trade with a lot of friends to do so.
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Only one is provided in the game, but you can trade one from someone else.
probably action replay but you can just buy a firered leafgreen sapphire or ruby and just save the masterball you get from them and give them to one of your Pokemon and migrate it
There would probably only be 3 ways to get masterballs. 1. The team aqua hideout. 2.The Pokemon lottery corner. If you win first prize, you get a masterball. 3. You can probably use a gameshark. I'm not too sure about that though. Anyway, that's cheating.
One masterball is in Pokemon emerald. To find out more go to my site called www.miquel.ning.com
You don't, as there's only one Master Ball in the game. You can hack to get more, but otherwise, there's no way to do so normally.