315 resistance is the cap for fire resistance. Any more and you're overdoing it.
When you are in fire resistance gear, the only thing you will want to worry about is fire resistance and stamina. Don't worry about if it's plate, leather, cloth -- when you're being hit with elemental damage, it doesn't matter how much armor you have.
With that said, the first thing you want to do is kill Lord Incendius in BRD. He drops fire resistance bracers of all shapes and sizes. They all have 10 fire resist on them, with various stats. Just use any ones you find with stamina on them.
You'll want to farm these:
http://www.thottbot.com/index.cgi?i=35816Many warriors overlook this, but if you're a skinner, farm for black dragonscales and get some black dragonscale armor crafted.
There is a 20 fire resistance cloak from UBRS, the Pyromancer guy. UBRS may also interest you since there is a helm that can drop there with a random enchantment on it, one of which can be, of fire resistance.
Finish your Onyxia key quest, and there's another 15 FR.
surprised