You can only get another masterball if a friend trades you a Pokemon holding the masterball they obtained.
No unless you have a game shark
You can only receive one master ball in leafgreen version its obtained from the president of Silph co after defeating team rocket and there boss.
you can either use a gameshark or action replay system. another way is to have another game card and complete the game to have a master ball and then transfer it to you game
Master Ball
after you beat Giovanni the second time talk to the man on the couch and he will give you a master ball
Only one
use a master ball on it
Have a friend trade you a Pokemon holding the master ball they received.
u cant
There is only one
You can get only one Master Ball in LeafGreen (as in every Pokemon Game) in the Silph Co. by Giovanni. Another way to get more Master Balls in LeafGreen, is to hack your game using Action Replay (if your game is in a Game Console, because there's another way of hacking Pokemon games if they are in your computer). Hope I helped :)
You should get the Master Ball by talking to the President of the Silph Co. in Safron City after you saved him by defeating Giovanni.