Once you get to go to the safari zone there will be a girl who will sell nest balls.
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Hearthrome city
You can buy net balls at the mossdeep city pokemart.
Go to Pastoria City and there will be two people at the counter. Press A in front of the guy on the left and he sells dusk balls, quick balls, and i think net balls.
Well if you go inside some poke'marts you can buy them from there. Hope this helped!!
In Pokemon Soulsilver, you can get a Net Ball from Johto in Goldenrod City. You have to win it from the Goldenrod Dept. Store lottery.