How to create Large Brick of High Quality Ore
Large Brick of High Quality Ore is used as an ingredient in a few different Smithing recipes as is even used in Mass Enchant Steel/Enchant Steel. There are very few NPCs who actually sell this item in the game, all of which you need to complete a quest (or someone does) in order for the vendor to even stock the items.
If you'd like to spend your time checking the vendors instead of doing the simple combines required to make this item, I recommend you head over to Allakhazam and read the comments for this item. There's a bunch of information there about the NPCs that sell this.
However in my experience you can't find these items sold from vendors. The only way to actually get a Large Brick of High Quality Ore is for you to take Fine Steel Weapons and combine them with a Water Flask in a Forge. This will break down the weapon into an ingredient you can use in Smithing. You'll find out more about this below, first, let's take a look at the two Smithing combines you can do to actually make a Large Brick of High Quality Ore.
Take these two items and combine them in a Forge to create a Large Brick of High Quality Ore.
Take these four items and combine them in a Forge to create a Large Brick of High Quality Ore.
In order for you to obtain the Small Brick of High Quality Ore or the Block of High Quality Ore you'll need to break down Fine Steel Weapons. I'd recommend you farm the Everfrost Giants or Rathe Mountains Hill Giants for Fine Steel Weapons.