*Possible* spoilers. Text is nice and small, so just copy-paste into notepad if you want to see it.
First, you need to get rid of the twilight on the Lanayru prvince (Hyrule Castle, town, Zora's domain, Lake Hylia areas). I don't know if you need to complete the third dungeon, but you might. You'll need about 2,000 rupees at the most, about 1,000 at least. I think you need more, dunno.
Ok. Go to Kakariko village, then you'll see some old goron in front of a building. He's not hard to miss, he sort of stands out from the rest of the gorons. Go into the building, then you'll see an old goron to the left of you. Speak to him, and he'll want you to fund the opening of the new Malo Mart in the castle town/Fix the bridge. One or the other, I forget which comes first.
Pay him, and hit the required amount of rupees required. After both the store is opened, and the bridge is fixed, exit the building and speak to the goron in front of it. He;ll ask you to deliver some hot spring water.
Your destination is the bridge to the west of castle town. A goron is laying there. You need to throw the water at him, in an alotted time limit before the water cools.
This is pretty hard, as there is enemies that just pop out of the ground, can destroy the crate, and generally reproduce faster than rabbits.
After you've done this, go to the town and look for the 'Malo Mart', and buy the armour for just under 600 rupees.
The armour makes you invincible while it is worn, but drains your rupees. Any hit you take will deduct from your rupees depending on the dtrength of the attack. When you're out of rupees, movement is impeded, and you can take damage.