On this page of Medieval Dynasty guide, you can find out where to look for foxes. Hunting three individuals is necessary to complete the "Into the Wilderness" quest.
Foxes are not as numerous as rabbits and boars. Fortunately, you don't have to travel too far to meet them. They can be found very quickly in the forest on the east of the village of Gostovia.
Hunting a fox is not hard. Usually, just one spear is enough to do it.