So the iconic shield can only be found in one place in the game, and that is deep inside the Hyrule Castle.
Hyrule Castle is at the northern edge of the Central Hyrule area, and once you get to the general vicinity of the whole place you will need to go to the back of the castle.
Look for a broken bridge there. Step onto the broken bridge; now turn right and look for a set of blue orbs off in the distance.
Then jump off and glide yourself towards those orbs.
Be sure to keep an eye out for a low-lying platform right before the orbs…make sure you land on it!
OK now watch out for the Guardian…oops, sorry forgot to mention that earlier, but fortunately, you can avoid the Guardian by sticking close to the wall at the top of the ramp to slide past him basically.
Once you do that there will be a door that goes inside the castle.
Go inside the castle and notice that there is a gate with a body of water below it. Use the Cryonis Rune to open up the gate and then use it to get across the whole thing.
Now you’re going to be in a dungeon with a bunch of prisoners (why else would they be there right?); look for a switch to open all the cells in the middle.
However, there are still some rooms with cell doors that can’t be opened by this method. So in order to get inside of them, you have to blast through the wall behind them with a bomb. Of course, this is all not necessary since it’s just farming for items right now. If you want you can go on ahead.
First save the game though in the dungeon.
OK now head to the side room at the very end of the whole area.
You’re going to see a bunch of bones lying around, and then you’re going to have a skeleton animate on top of you. That skeleton becomes a giant Stalnox.
Kill the Stalnox (obviously) with either shooting it in the eye with a bow and arrow or by just simply getting up close and personal with melee attacks.
When it dies its eye will drop out of its socket. Destroy the eye so that the Stalnox doesn’t resurrect itself again.
And now the creature will drop some loot/a chest…inside is the Hylian Shield!