During your quest in Skyward Sword you will be able to upgrade your shields several times using the treasures Link finds along the way. There are essentially three different shields you can find: the Wooden Shield, the Iron Shield and the Sacred Shield, all of which can be upgraded into higher quality. Here is a quick reference guide on how to find and upgrade all the shields in the game, and please note that you must visit the Scrap Shop in the Bazaar in Skyloft to perform the upgrades.
The Wooden Shield
The first shield Link will obtain in the game is the standard Wooden Shield. You can find this by talking to Owlan outside the Knight academy after you obtain the practice sword. You can also purchase the same shield for 50 rupees a the Bazaar if this one is destroyed.
A stronger version of the standard wooden shield.
Items required for upgrade: Amber Relic (2), Monster Claw (2), Jelly Blob (1)
The final upgrade to the wooden shield, improved durability.
Items required for upgrade: Amber Relic (3), Monster Claw (2), Tumbleweed (2), Ornamental Skull (1)
The Iron Shield
Unlike the wooden shield, the iron shield is immune to fire and is much stronger and durable, on the downside its weakness is electrical attacks. you can obtain this shield by purchasing it at the Bazaar for 100 rupees after completing the Skyview Temple.
A stronger and more durable version of the iron shield.
Items required for upgrade: Eldin Ore (2), Monster Claw (2), Ornamental Skull (2)
The fortified shield is the final upgrade to the iron shield and has improved durability and strength.
Items required for upgrade: Eldin Ore (3), Monster Claw (3), Tumbleweed (3), Blue Bird Feather (1)
The Sacred Shield
The Sacred Shield can be purchased for 500 rupees at the Gear Shop after you complete the Lanayru Mining facility. This new shield has a low shield meter, meaning weak durability, but makes up for it with being resistant to fire and electrical attacks. However it is weak against magic.
The first upgrade to the Sacred shield.
Items required for upgrade: Dusk Relic (1), Bird Feather (2), Ornamental Skull (1)
An upgrade to the divine shield, the Goddess shield is much more durable and has a larger shield meter for durability. It is also the first upgrade in which the shield meter can replenish itself.
Items required for upgrade: Dusk Relic (4), Bird Feather (3), Monster Horn (4), Blue Bird Feather (1)
The final and strongest shield in the game. You can obtain this indestructible shield by completing the Boss Rush mode. After you have saved the Thunder Dragon in Lanayru Gorge, you can return to re fight boss battles, once you have defeated all 8 bosses you will be rewarded with the Hylian Shield.