I guess you can't but qny
Hearthrome city
at mulch iland
You can buy net balls at the mossdeep city pokemart.
Once you get to go to the safari zone there will be a girl who will sell nest balls.
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!!
For Fire Red, I believe you could get 1 net ball in the game. One of them is from the fisherman south of Lavender City, and if you catch a magikarp and give it to him and it is big enough, he will give you a net ball.
Great Balls:fallabor town Net Balls:Mossdeep city.
Mossdeep City
go on Pokemon firered and leaf green serrebi .net
You can get net balls at most pokémarts like the one in sunyshore city and the pokémon league net balls help you catch bug and water type pokémon -hoped i helped you- ~Ninjachoco~
you need to go to one island and go in the Pokemon net center and talk to celio