For the most part, it's random loot. The one placed Resist Fire piece I can think of -- Ancient Helmet of the Unburned -- can't be disenchanted. Of course, that might also be the solution, since it grants 40% resistance. You get it in a dungeon during a College of Winterhold quest. Otar's also nice (30% resist against all elements).
But if you're not ready for some tough bosses, you can either get lucky with loot, or just visit armor sellers until one of them sells it.
Or you can create a potion, since this has a very specific, time-limited purpose. Any pairing of the following will work: Bone Meal, Dragon's Tongue, Elves Ear, Fire Salts, Fly Amanita, Mudcrab Chitin or Snowberries.