Rare candies cannot be bought in any of the Pokémon games, but Generation III introduced a way to gather as many as you need using the Pickup ability. This method relies on probability, so it can take hours of grinding.Continue Reading
The first thing you need is to have a Pokémon with the Pickup ability in your party. In Pokémon Ruby, only six Pokémon can have Pickup: Meowth, Aipom, Teddiursa, Phanpy, Zigzagoon and Linoone.
Make sure that your Pokémon with Pickup is not holding any items, then go around battling other Pokémon. After every fight, there is a 10 percent chance that your Pokémon will pick up an item and another 10 percent chance that the item will be a Rare Candy. The Pokémon with Pickup doesn't need to fight to use its ability after a battle.
Every time your Pokémon picks up an item, put it away and keep looking for battles. Continue this process until you gather 99 Rare Candies.