There are several ways to answer how to get more Pokeballs in Pokemon GO.
Pokemon GO, like any other Pokemon game, revolves around the capture, care, and combat of unique creatures that we call Pokemon. In this world and, now, ours, Pokemon can be caught using spherical shaped devices called Pokeballs—of which there are several kinds. Each different type of Pokeball features a distinct advantage when trying to add other creatures to your team.
How to Get More Pokeballs in Pokemon GO
One of the quickest ways to get Pokeballs is to frequent PokeStops. Spinning stops and gyms will give trainers a handful of items to help them on their quest including Pokeballs, Great Balls, Ultra Balls berries, and occasionally revives. You can spin a Pokestop multiple times in an hour, so it may be worth your while to camp a populated area if you're low on supplies.
Another way to get Pokeballs is to meet goals. Whether this is your weekly walking goal, research breakthroughs, or quests, it's not uncommon to be rewarded with various types of Pokeballs upon completion.
The final way to get Pokeballs in Pokemon GO is simply to buy them. You can get a pack of about 20 for 100 PokeCoins at any time. PokeCoins can be earned by defending gyms. However, if you follow the advice stated above, you likely will never need to spend your coins like that.