You can find recruitable Gastly and its evolutions in Sky Stairway.
You really need wondermail for that?!
You can find them commonly in Sky Stairs. You need to get the wondermail code for it.
You have to buy it in a shop
anywhere
You can find Jolteons at Spacial Rift.
You find a chimchar, train it, evolve it.
Find them all.
you find darkrai in the dark crater!
No. You'll have to find some more codes specifically for explorers of sky. (Sorry I don't know any. I only have Time and Darkness.)
i do not have explorers of sky
in any dungeon
yes