You have to buy it in a shop
Find it.
kecleon shop
Any Pokemon that has an evolution can evolve. Some Pokemon may need some sort of item like a Thunderstone to evolve, though.
wonder mail
receive a thunderstone and when it asks if you want to use anitem say "yes" and then choose thunderstone
i do not have explorers of sky
Yes, Arceus and Shaymin are both featured in Pokemon Explorers of Sky.
Why are you playing explorers of sky? You should be playing the real Pokemon games.
The sky stairway
Unfortunately you can get Arceus in Explorers of sky :/ The closest you get to seeing it is a statue...
You have to evolve it from Eevee. If there is a thunderstone in that game, use the thunderstone and Eevee will turn into Jolteon. If there is no thunderstone, then you must evolve Eevee by training it, but it might turn into Flareon or whatever the water pokemon's name is that it turns into. You can also trade it with yourself to a game that has a thunderstone and evolve Eevee that way, then trade it back.
Its not in the explorers of sky
To get Latias in explorers of sky you need the place called sky fissure 10F & a thing called Secret stone plate or Enigma part in bag. Also to get Lotios in explorers of sky you need sky fissure Deepest part B5F