Well, as a Master Ball is extremely rare and it is has a 500% catch rate, there is only ONE Master Ball in the whole game so you can't get a Master Ball more than once without cheating!
You trade Pokemon from friends that's Pokemon is holding a master ball
maybe having a Pokemon with pickup as an effect may pick a master ball but it is extreamly dificult to pick
you make a lot of money and buy another one. knock knock knock
With cheats or action replay.
there is no more masterballs in Pokemon platinum but not if you use cheats
There is only one master ball in every poke'mon game per file. Therefor it is futile to even try to get more without using cheats. You could use a games shark or an action replay to achieve 999 master balls.
You can't without using cheats.
if you want another want you have to hack it or use cheats
you can only get one in the game without cheats
if you have other GBA games then you can play the GBA game on the DS until you get a master ball, and then you can give the master ball to a Pokemon and send it to the pal park on the DS game or you can use cheats xP
give one to a Pokemon and trade the Pokemon with the master ball.
more like 493 master balls. its a cheat on the AR look up the code for it. its called Pokemon modifier. you can keep refilling it though.