Firstly you can buy them from the shop. That’s the quickest way. Secondly you can find them by visiting the blue markers on the map, clicking on the marker when in range and swiping your finger across the picture of the location to spin it around. This will produce poke balls. Click the poke balls before they float away to add them to your inventory.
You can only get a few poke balls at a time with this method, but you can have a respin if you walk away and come back to the same spot when it turns blue again. Note: spinning it will turn it purple for a while.