You can't buy rare candy in fire red. You can find them all over the region though.
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You cannot purchase rare candy anywhere in the game.
by cheat
You can't buy rare candies in fire red. You can only find them, if you don't use cheats.
You can get three rare candies in Pokemon ruby sapphire and emerald
The item duplication glitch (or Rare Candy glitch) can't be done in Pokemon FireRed version.