Mages, priests, and warlocks can only wear cloth. You must rember that the only benefit to other armor types is the bonus physical damage reduction you take while wearing them (more armor). By the time you're level 80, that's essentialy all you'd be missing, becuase mages stack intellect, stamina, spell power, critical hit rating, haste, hit rating... ect. All gear, aside from tanking gear, gives more or less the same amount of stamina, furthermore arcane mages can improve their armor with talents, and mages of other specs may have multiple damage migitation spells, (Frost mages with a shield, arcane mages with a mana shield). Really, a good mage isn't dead till they're out of mana.
You can get armor from vendors, the auction house, quests... just remember, in the lower levels, you want intellect and stamina, maybe spirit can replace stamina if you're confident.
As for the answer directly below me, you're misguided. Let me explain: Cloth: Warlock Priest Mage - Leather: Hunter and Shaman TILL level 40, Rogue and Druid permanently. - Mail: Warrior and Paladin TILL lvl 40, Hunter and Shaman FROM level 40 on. - Plate: Death Knight and Warrior and Paladin FROM level 40. Furthermore, mages CAN use "Held in Offhand" gear as well as 1handed swords.