Cyrus gives you one. but apart from that the only way to get other master balls is by winning the lottery
If you want to get more than one you will need to get two ds's or borrow one, have more than one Pokemon ds game (Pokemon pearl, diamond or another platinum) and you need to get one of Cyrus then trade a Pokemon (I'd recommend a low lv Pokemon that you don't care about and make sure you trade your favourite Pokemon onto the other game) and make sure its holding your master ball and then delete your save on Pokemon platinum (that's why you need to trade your favourite Pokemon) and start a new file then get up to the part where Cyrus gives you a master ball and repeat this over and over again (it does take a while but its worth it) when you do that you can get more than one master ball and when you are happy with the number of master balls trade one Pokemon onto platinum whilst it's holding the master ball and repeat until all your master balls are on platinum or you can 999 master balls of the action replay ds using one of the cheats enjoy ;)
he never gave me a master ball :(
you can not get a master ball in Pokemon platinum only the one professor Rowan gives you
cheats
You can't.
Action replay
no
cyrus gives it to you
Yes there is a code for 999 master balls in Pokemon platinum
you have to have Pokemon platinum and a action replay then if u want just give your Pokemon master balls and trade them to games you want to have master balls
Pokemon lotery and action replay
Go to your house and talk to your mum and ask in the microphone can i have master balls. don't listen
Cyrus gives u one
l and r