These are found in fetchquests where certain NPCs will ask you for a mithril item, in which he or she will exchange it for the red brick. Below is the complete list of red bricks, including the requirements needed for each brick, the level where it can be found, what the red brick does, and the location of each NPC who will provide you the red brick. Refer to the Mithril Items
Guide on how to acquire the particular Mithril Item required for the Red Brick
#1: Pooh Studs - 25000
Requirement: Mithril Rope - Amon Hen
Use: Makes studs come out instead of pooh when you press action button on an animal.
Found: At the Northern part of Bag End, at the back of a Morgul block.
#2: Disguises - 50000
Requirement: Mithril Cluster Bow - The Pass of Caradhras
Use: Makes Characters
wear funny-looking mustache and glasses.
Found: At the house from the right of The Hall of King in Edoras.
#3: Boss Disguises - 100000
Requirement: Mithril Music Horn - The Black Rider
Use: Gives alternate colors to enemy bosses.
Found: Explosives needed. In an island located west from the Pass of Caradhras.
#4: 8-Bit Music - 200000
Requirement: Mithril Climbing Shoes - Mount Doom
Found: Western tower of The Black Gate.
#5: Studs x2 - 250000
Requirement: Mithril Gloves - Warg Attack
Found: Under Elrond's home in Northeastern Rivendell.
#6: Studs x4 - 1000000
Requirement: Mithril Fishing Rod - Track Hobbits
Found: Western area of the river near Amon Hen.
#7: Studs x6 - 4000000
Requirement: Mithril Top Hat - Battle of Pelennor Fields
Found: Southern area of the Hall of The King in Edoras.
#8: Studs x8 - 8000000
Requirement: Mithril Shield - The Paths of the Dead
Found: Dwarf located atop a steep slope east of Helm's Deep.
#9: Studs x10 - 10000000
Requirement: Mithril Squeak Sword - The Secret Stairs
Found: Eastern part of Mount Doom.
#10: Quest Finder - 50000
Requirement: Mithril Trowel - Blacksmith in Bree
Use: Makes pointer arrows show up. Red arrows lead to Red Bricks. White arrows lead to Mithril Bricks
Found: Front of the Prancing Pony.
#11: Minikit Chest Finder - 50000
Requirement: Mithril Stud Mallet - The Dead Marshes
Use: Makes pointer arrows show up. White arrows lead to chests.
Found: In Barrows Downs at northern part of Weathertop.
#12: Mithril Brick Finder - 75000
Requirement: Mithril Boxing Gloves - Osgiliath
Use: Makes pointer arrows show up. White arrows lead to where Mithril Bricks
Found: Southwestern corrner of Bree.
#13: Character Studs - 25000
Requirement: Mithril Fire Bow - Helm's Deep
Use: Makes enemies drop studs when you hit them.
Found: Southwestern part of Minas Morgul.
#14: Attract Studs - 100000
Requirement: Mithril Carrot Bow - Taming Gollum
Found: Northeastern part of the exit of Moria, at the back of the rocks that should be pulled down.
#15: Action Assist - 25000
Requirement: Mithril Whistle Sword - Bree
Use: Makes actions that require you to hit buttons much easier.
Found: Western area of Cirith Ungol.
#16: Fast Build - 100000
Requirement: Mithril Music Boots - Weathertop
Found: Near the waterfalls located Southwest of Isengard.
#17: Fall Rescue - 75000
Requirement: Mithril Shockwave Axe - Prologue
Use: Restarts you from your last location before falling off an edge.
Found: Near Galadriel and Celeborn in Lothlórien.
#18: Mithril Hearts - 75000
Requirement: Mithril Headdress - The Mines of Moria
Use: Makes all Characters
able to take eight hits.
Found: Northern part of The Dead Marshes.
#19: Regenerate Hearts - 100000
Requirement: Mithril Ice Bow - Cirith Ungol
Use: Makes hearts regenerate constantly.
Found: Berserker Bombs needed. Inside a cave South of Weathertop.
#20: Invincibility - 500000
Requirement: Mithril Tinderbox - The Black Gate
Use: Grants invincibility, making hearts don't disappear at all.
Found: In the path between between the Dead Marshes and Osgiliath.