I started with a female Froakie in Pokemon X, but I did not know you had to have a male to breed. I finished the game but I want to breed my Greninja. How can I do that?
Alternatively, as your Greninja is female, you can breed it with a male of a different type of Pokemon, so long as the male is in the same egg group as the Froakie evolutionary line.
Froakie and its evolutions are in the Water 1 egg group, which contains 108 Pokemon. For example, Poliwag, Psyduck and Marrill are all in Water 1 and relatively accessible to catch in-game.