Instructions
Find a rare candy on the fourth floor of the Olivine Lighthouse. Head north and you'll find the rare candy.
Go to Violet City after teaching a Pokémon the surf ability. Surf through the water in Violet City, and eventually you will randomly find the rare candy hidden in the lake.
Head to Mt. Mortar with the flash, surf and waterfall abilities equipped. Enter the cave and use flash to light your way and head through the water until you reach the waterfall. Use waterfall to climb to the next level. Bear left and you'll find the rare candy.
Go to Route 27 (the road connecting Johto and Kanto), get into the water and surf toward the east, and you'll find the rare candy.
Go to the building with the green roof (Pokémon Fan Club) in Vermillion City. Talk to the man at the head of the table inside and he'll give you a rare candy.