It's random. You just have to keep adding people on your friend list and hope that they have a water safari and hope that Feebas is a part of their three pokemon. Mine is water, however my pokemon are Octillary, Wartortle, and Frogadier
Since this question was answered I'm locking this. In the future please ask quick X&Y questions like this in the pinned Quick Q&A thread.
There are also threads open for sharing friend codes for the friend safari, so feel free to visit those as well~