There are no robes in the game; you may have to stick to cloth(?) or leather.
You can find or buy mythril plate armour (strong armour with no strength requirements), which is good for wizards, except of course for not fitting into the classical magic user build.
If you are more put off by how your character looks in heavy armour than wanting to play by rules from other games ( <img src="/ubbthreads/images/graemlins/silly.gif" alt="" /> ), there is a silver breastplate you can find in the Dark Forest. This can be upgraded (see below), after which it is invisible on your character.
Silver Armour and Treasure Maps (Spoilers) - you can find the treasure maps and the cave yourself, though will need these instructions to continue to the easter egg area