There is no natural method of obtaining unlimited rare candies in Pokemon Fire Red (or Pokemon Leaf Green). The only way to do this is by using a Pokemon Modifier code on a PC emulator ROM.
In the first generation of Pokemon games, there was a glitch that allowed the player to get an unlimited amount of any item. This was done by performing a specific sequence of events to encounter the missing data glitch (commonly known as "Missingno" for short). However, this glitch only occurred in the Pokemon Red and Blue Versions and was fixed in subsequent generations. In fact, all versions after the second generation are completely devoid of any inherent glitches or cheats.