There are 3 ways to get PokeBalls in the game: buying them in the Store, getting them at PokeStops & by leveling up. Simplest one is the most obvious one. Buying them at the store. You're given 3 options to buy: 20 PokeBalls for a 100 coin, 100 PokeBalls for 460 coin & 200 PokeBalls for 800 coin. Obviously the 200 pack will give you more bang for buck. Similarly to purchasing coin where the more you buy, the better price you get. Another way to get PokeBalls is by visiting PokeStops. Not counting the store, this is the most effective way to stock up on PokeBalls. Usually I get around 4 per PokeStop in average. This means a simple walk in my neighborhood will get me more than enough for a full day worth of catching.This might not be the case for you Last and least important way is by leveling up. When you get those 15 or so PokeBalls it's awesome but it's not close to enough to reach the next level so ultimately you're gonna be forced to use one of the first 2 options anyway.