In Pokemon Go, you can capture high CP Pokemon easier by using Great Balls which cannot be bought in the store and you have to get them by yourself. Check out this guide to figure out how to get Great Balls in Pokemon Go!
Pokemon Go Great Balls
You can toss Great Balls whenever you get them. However, because they are much rarer than Poke Balls, you should save them for red or orange captures until later in the game.
As battling for Gyms, getting Great Balls in Pokemon Go is level based. When reaching level 12, you will receive 20 Great Balls which you can use later. At level 13, you will get 15 more Great Balls as the rewards, in order to read more about level rewards, please take a look at this page! Basically, from level 12, you will get a bunch of Great Balls after reaching the next level.
After unlocking the first Great Ball set, you can start getting them from Poke Stops if you are lucky. The Poke Balls are guaranteed but the Great Balls are random. Sometimes I can get several Great Balls from the Poke Stop but I usually just get normal Poke Balls and Eggs. Therefore, be sure to save the Great Balls until you find really tough Pokemon.
Surely, I will update if I find a way to get Great Balls quicker! At this time, traveling by bus/train is still the fastest way to gather loads of Poke Stops quickly.
Please comment If you have any advice or question!