If you've been vigilant at catching all the Pokémon you come across in your adventures across Kalos, then you'll probably end up with a Pokédex of 300-400 Pokémon. You can build it up through more trading and battling other players, but you can also visit the new Safari Zone, redesigned for Pokémon X and Y.
Dubbed the Friend Safari Zone, this zone gets populated depending on the amount of friends you have. With each registered friend code, you'll get a new square zone to explore, each one populated with various Pokémon depending on the element listed on the Safari select zone.
You'll find the Safari Zone in Kiloude City, which will be accessible after you completely beat the game, watch the credits, and visit Professor Sycamore in the Lumiose City Train Station. The Safari Zone house is located in the top most right corner of that city.
There will be up to three different Pokémon available to catch, though only two if the friend hasn't beaten the Elite Four. The best part is that the registered friend doesn't even need to own or play a Pokémon game in order for a Friend Safari Zone to be available. If the player is online at the same time you're exploring their respective Safari Zone, the Pokémon will get Hidden Abilities as well.
The difference between Safari Zone in X/Y and previous games is that there is no timer to worry about, nor are there special Safari balls you have to use, allowing you to use any Pokeballs you have in your inventory.
