We have bow, one handed, 2 handed weapons damage boost from armor via enchanting or get it from drops or even buy those from NPCs along with talent perks that boost damage up.
For mage we have only perks that boost 50% damge for each elements. My character is lv70 and i never see any items that has spell damage boost for mage on armor. Does anyone ever see it on your journey?

It does not exist as an enchant or spell, but you can brew a potion that increases destruction damage.

Just a tip, to increase (double IIRC) the damage of fire destruction spells, you first need to get the flame perk which fears the enemy with fire magic. Then get the illusion spell that boosts fear effects, gives fire destruction a big boost.

yeah perk illusion to make your destruction more powerful!!! duuuuh...

** The silent sound of 28 forumgoers actually trying to perk Illusion to increase destruction damage **