If you haven't come across a friendly player who will trade you one of his starter Pokemon, nor have you found a Ditto so you can start breeding them, this little tip will surely come in handy.
After you beat the Elite Four and you become the Pokemon League Champion, make sure to stay for the entirety of the credits, so you can save your game and continue on your journey to catch 'em all. You'll start back in Vaniville Town. After leaving your house, you'll be greeted by Shauna, the girl who picks the starter Pokemon that you're stronger against. She'll inform you that it had an egg and that you can have it, giving you access to a second starter.
If you picked Froakie, she'll give you Fennekin
If you picked Fennekin, she'll give you Chespin
If you picked Chespin, she'll give you Froakie
So there you have it, yet another bonus to completing the game. Now just go catch Zygarde, Mewtwo and one of the Legendary Birds.
