To evolve an Eevee into a Glaceon or Leafeon, you must be standing next to Ice Rock or Moss Rock when leveling it up. Ice Rock can be found first; it is located in the deepest part of the Frost Cavern (the cave is not too large). Moss Rock is found closer to the end the quest to defeat the Elite Four. It is on Route 20, between Snowbelle City and the Pokemon Village. Enter the area from Snowbelle to immediately see it.