You have to go to the Safari zone near lillycove city.There you will find a natu but you need pokeblocks you will find pokeblocks at slateport city at the Pokemon contest place in the place where you check in there will be a little girl that will give you pokeblocks.
You can get natu and xatu (the evolved form of natu) in the safari zone near fortree city
in the saffari zone
level 25
you have to evolve natu at lv.54
You can catch a shuckle, wooper, heracross, phampy, and I think swirlfish or whatever that spike puff fish is named. Regular dodrio, pikachu, natu, xatu, magicarp not that it matters, goldeen, etc.
in ruby, saphire, and emerald, you get it from the safari (or evolve ith from natu which is also in the safari).
nauto envolve at level 25
You cannot catch Natu in Pokemon Black. You will have to trade for one or transfer one from a DS Series game.
They swarm occasionally on route 224
There's an entrance to it on Route 32. You can't catch a Natu there, however; Natu are found in the Ruins of Alph and Route 28, and on Mt. Silver if you use Headbutt.
Catch a Natu in the Safari Zone, and raise it to Lv. 25.
There is no way to get it (at least at Pearl), but you can migrate it from Pokemon Ruby, Sapphire and Emerald and maybe LeafGreen and FireRed. Or you can migrate Natu.