once you have the 8th badge you will be able to get it
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first defeat Clair (the last gym leader in jhoto) then when you exit professor elm. will call you saying he has a special thing for you go to new bark town he will give you the master ball
Professor Elm calls you as soon as you leave the Dragon Den in Blackthorn City. Fly to New Bark Town, and he'll give you a Master Ball. This is the only Master Ball in the game. The only way to get another one is to hack or trade to another game having that game's Pokemon holding the Master Ball. EDIT: You can also get a Master Ball by winning the lottery on the Lucky Numbers Show.
In Pokemon Soul Silver after catching Lugia go to New Bark Town and talk to professor Elm he'll give you a Master Ball after that travel east and surf to route 27 when you get to land you will have entered the Kanto region (Whoo Hoo)
go to elms lab he gives you a master ball go to ecruteak city battle kamino girls go to whirl island find lugia capture surf from new bark town and battle elite 4
In Johto: New Bark Town, Azalea Town, and Mahogany Town. In Kanto: Pallet Town.