Blue (The eighth gym leader) is waiting on Cinnabar. He won't battle you unless you've beat the rest of the gym leaders
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In Johto you can find it in the Safari zone and in Kanto you can find it in Rock tunnel.
he is at the sea form islands
in eutreaka city with the fourth gym leader morty, go to the pokemon center and he should be there
If you mean how to consider yourself as beaten the kanto region in pokemon soulsilver, then you should find zapdos,moltres, and articuno. You should also beat Red (Ash Ketchum). Once you do that, you can catch Mewtwo.
You can find a department store in any town exept for pallet and the island where blaine the gym leader was.