You need to get the official Pokemon full pokedex guide vol 2
in eternal forest with poke radar
you have to trade spheres for it
You can find a Houndoom in diamond, you find it in route 214 or 215 using the poke radar. well hope this helped.
in the poke mart deparment store
you can't,you have to use the poke radar on route 206 to find larvitar
You find Kirlia on Route 203 and 204 using a Poke Radar.
After the game, use the Poke Radar north of Jubilife City.
look in a poke man book
you open a diamond with another diamond stick a ball in it ,put it back together and poke it!
In Solaceon Town, right next to the poke center
You find Rare Candy's in Poke Balls through your adventure.
You cannot find Gardevior in the wild on Pokemon Diamond you will need to evolve a Kirlia. To catch a Kirlia you will need the Poke Radar. Now use the Poke Radar on Routes 203 and 204 and a wild Kirlia should eventually appear.