The most common and likely way of getting the rest of the Pokémon into your game is through the Spot Pass feature. Every day, you will get up to three Pokémon sent automatically through the Nintendo WiFi Connection. These three Pokémon are completely random Pokémon that you don't normally have, but there are ways to skew the Pokémon you get
For each Pokémon, there is an AR Marker. This marker is a simple marker which each Pokémon uses in order to appear in the Augmented Reality Viewer part of the game, but if you have yet to obtain the Pokémon, it will register it as a sticker in the sticker book. Now, this Pokémon has a higher chance of being chosen as the Pokémon obtained through the Spot Pass feature.This website should help you I guess.See this to see what Pokemon are available and how.