There are three cafes around Kalos that you have to pay to enter. Each possible seat you choose will allow you to talk to patrons who will show you Pokemon for your Pokedex. The 100 price option will show you one Pokemon. The 1,000 price option will show you two Pokemon. The 5,000 price option will only show one Pokemon, but it will be guaranteed to be one you haven't seen. You can only do each one once, so use it only as a last resort for pokemon you can't find.