How do I get more rare candys and masters balls?

Cheat or get a friend to Trade you a Pokemon holding one of them.

You can only get one Master Ball but keep a look out for Rare Candies.

Rare candys you just find and for MBs you need a cheat divice.

I heard that if you want more masterballs, you Trade with a friend whose Pokemon has a mb, and when it saves your friend shuts off their Gameboy while it's saving. I haven't tried it, though. Too risky.

Here's the best answer that noone has mentioned yet. If you find a way to Trade with Ruby/Sapphire then on Ruby/Sapphire you can use Linoone or Zigzagoon to pickup random items as you are walking around, most of the time I get revives and rare candies, then trade them, simple. As for the Master Ball, you can win one at the Loto, so I'm sure 1 day a year you can get one. Good luck.

You can use the old man glitch. You talk to the old man in the 2nd town you visit agree to let him show you how to catch Pokemon switch mb or rare candy to 6th slot Fly to cinnabar swim along coast.

You can use the old man glitch. You talk to the old man in the 2nd town you visit agree to let him show you how to catch Pokemon switch mb or rare candy to 6th slot Fly to cinnabar swim along coast.