I want one of these two Eeveelutions as my last Pokemon in my in game team. I don't really need a sixth but why not add one just for the sake of adding one. Now, I'm not gonna add the other Eeveelutions because I hate Espeon and Umbreon, I already have an electric and water type and Flareon sucks. So, are these two post game only?
Yes, sorry. The Mossy Rock is in the Pinwheel Forest, only accessible after beating the game. The Icy Rock is in Twist Mountain, which is blocked off by Marshal until you conquer the Champion.
Source: Copious experience
Hope I helped!