i don't now were to find it but gengar is good
Yes by trading.
There's no dark Pokemon, that's only in the movie. If you mean Dark-type Pokemon, that's a different story.
in the dark cave near blackthorn city
You buy it with points at the Pokeathlon Dome.
dark type and some fire types
Darkrai is only available through Pokemon events
His name is Will. He is a master In phsycic type Pokemon. Bring some dark and bug Pokemon to defeat him! good luck!
you sell it in a shop
route 206, stunky, also mt. silver larvatar and sneasel.
if this is the swirl islands . . <-- . . go to this one
Fire, dark, ground (if no Levitate), ghost
I recommend the use of a Dark or Psychic Pokemon or a Pokemon with those moves.