you just look up natu in the Pokemon and click 'AREA'. and it tells you where to find him. but I'll tell you anyway. go into union cave and then go out to the outside. instead of where the place you enter ruins of alph normally, you should be in a small enclosed space. there is a trainer there so battle him. there is a patch of grass there as well. you can catch smeargle and natu there.
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Natu evolves at level 25.
I don't know if you can get it in crystal, but i know you can get it in Gold and Silver versions. So you go to Union Cave. then you take a secret path (involves surf) and you go to this small patch of grass. In there you can find Smeargle (I think that's how you spell it) and Natu. Again i'm not sure if you can find them in crystal.
You can get natu and xatu (the evolved form of natu) in the safari zone near fortree city
To get Natu you have to pass through the door in the right in the mansion in route 212.
You can get natu at the ruins of alph in heart gold and soul silver