You can only get one if you trade for it or catch a Frogadier at a friend safari.
Those are the only ways.
It is one of the 3 Pokemon in the 3rd slot, meaning the person whose safari it is must have defeated the league and been online at the same time as you in order to unlock it.
Make sure it's a female one so the offspring from breeding will be a Froakie, unless you're breeding with a ditto.
Source: Friend safari page and knowledge
Hope I helped.