By hacking then yes not hacking no
you have to get dexoys
use an action replay
you find it in the team aqua hideout
Win the lottery at the lilycove dept store.
You can't
You can cheat via Action Replay, Game Shark, Code Breaker ect. OR of course you could use the Pokemon Emerald Cloning Glitch to Duplicate Master Balls and trade them to Pokemon Ruby.
you cant
yes
Just one
You can't buy them
Use a GameShark.
You can buy Game boy Advanced cheat games. You can then Click on the 999 Master balls cheat.