Where do you get pokeballs?

read title.

At a pokemart

In pokemarts, the helios city department center (second floor, middle clerk), in hidden grottos, and just laying around in some areas.

I just started the game, where is the nearest pokemart?

In the pokemon center, go up the stairs and you can talk to them.

Oh. talk to the clerk in the front on the pokemon center for 5 free samples and buy more at the top floor of the center.