Madrigal is quite right. Armor for mages is silly. Spell proficiency is far more important for spellcasters. You'll find suitable protection for mages as you go through the game. Elytharra, for instance, has bracers for sale that your mage can wear.
I've found that, most of the time, keeping spellcasters in the rear keeps them out of trouble. Also consider protective spells like mirror image and protection from arrows. That's why they're available
Nightowl2