In Pokemon diamond and pearl, there are 5 ways to get master ball:
1)win it from lottery in jubilife(the lottery number should match the ID number of any of your pokemon on team or in box)
2)go to the galactic HQ in veilstone and find it in there
3)use x999 masterball cheat on action replay
4)migrate Pokemon holding a masterball from a GBA game
5)put a good Pokemon up for trade in WIFI and ask for a masterball

for number 4, if you migrate a Pokemon from emerald holding a masterball, you might want to clone it first, and this is how(hint: you can clone up to 5 Pokemon [INCLUDING LEGENDS] at a time and you can also make all of them hold items to clone):
1) Go to the battle frontier
2) Go to the battle tower
3) Log on onto the PC
4) deposit the Pokemon you want to clone into any empty box but make sure that they are all in the same box
5) Exit the PC and Save the game
6) Withdraw the same Pokemon you just deposited
7) Go to the lady to your far right(the battle tower lady closest to the PC)
8) talk to her and select any battle options, but don't change your mind
9) Select any 2 Pokemon that are allowed to be used in the battle tower
10) when the lady asks if you want to save the game(you might notice a pause happens before this, if this doesn't happen, the cloning might not work), turn off the game
11) on your game again, and it shouldn't say anything about corrupt game data, and when you load up the game, and check your team and the box you used for cloning your Pokemon in,and you should see the same Pokemon in your team and in the box(excluding one Pokemon from your team, the one you didn't clone)