First (after you've got a radio card) turn your radio to channel 10.5 (only from 6:00pm-12:00am)you will hear a channel called Buena's Password. Go to the radio tower and talk to the lady on the 2nd floor. She will give you a blue card. Next talk to the lady again she will ask you what is the password. If you enter it correctly she'll give you a point on your blue card. The lady next to her give's you stuff for your points. Save 3 points on your card to get a rare candy
And here are a few rare candy locations(Rare candies are well...rare!):
1) go to cinnibar island. U know where blue is? If he's not there, press A on the spot he was standing on before he left.
2) go to violet city. There's this pond somewhere in the north part of the city. Use surf and go forward until you see a pokeball.
3) In KantoWhen you first go there, right under the cave, there will be a long stream of water. Surf till you find an item in a piece of land. It is Rare Candy.
4)Another one is in Kanto and you listen to a guy in vermillion city tell a story, then he gives you it.
Added by: Pravinj Apr 25th 2012, ID#5395