To get a Leafeon, first you must have an Evee.
Evees can be found on Route 10, in the second area of yellow flowers, outside of Cyllage City. Evees can also be found in the grass patches right before you enter Geosenge Town.
Once you get your beloved Evee, go to snowbelle city and enter the Winding Woods, or Route 20. The leaf stone you need to train your Evee with should be in the center of the first grass patch you see on Route 20.
After you find the leaf stone, train your Evee around it until Evee levels up. When Evee levels up, it should turn into Leafeon. (The leaf stone must be insight while training Evee.)