Well, with rare candy, if you feed it to a Pokemon, it levels that Pokemon up by 1 level. These candies are very rare (duh, it's in the name) so I suggest you use it wisely.
i will tell u but ..... ok.. go in cinnnabur island and go to day care and say no when she asks go to PC and take out ditto or pidgi say yes and then surf a round the island u will get a pokeball in...
Rare candy can be found in a lot of places: Caves, around routes, hidden around kanto. Also if a Pokemon has the ability pickup it can occasionally find rare candy.
you can get them throughout the game, but mainly in the team rocket hideouts, you should have 2-3 by the time you have 5 badges
Where You Can Find 5 Rare Candies.
In Mt. Moon, there is one on the...