Question

I know that it is in pewter city but where and how do you get one?

Answer 1

You dont find them in Pewter city you take the Dome Fossil to Pewter city and get it revived into Kabuto. You can find the fossil by smashing rocks in the Ruin of Alphs.

Comments

armor fossil gives you shieldon.  Dome fossil gives you kabuto

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Whoops ill fix that

Answer 2

You can find the dome fossil after randomly smashing rocks in the ruins of alph. Afterwards, just bring it to pewter city museum and restore it.
Source: http://bulbapedia.bulbagarden.net/wiki/Dome_and_Helix_Fossils