You find them throughout the game, or by use of an Action Replay. If you want an exact location you should Google "Location of rare candies in Pokemon Diamond."
get a action replay
Try XKs Unlimited at www.xks.com
ricolu a Pokemon you have probably never heard of as its realy rear
prany i dont know
chach a rear leaf Pokemon that fly
One of the rear Pokemons is Whalelord...
find them
you can yous stones or if you have hacks you can yous 999 rear candys
train the Pokemon's give it a puffin or a rear candyto make it envolve
Where you buy dildos you can get some rear candy ^u^
Rare Pokemon that can be caught in firered are: Moltres, Zapdos, Articuno, Mewtwo, Raikou/Suicune/Entei (depends on which starter Pokemon you choose).