Erdur Ore is very rare. It can only be found deep underground, and almost all of it was once minde out by gnomes. Gnomish masters learned to smelt it into a metal to create excellent armour. Ancient recipes and frames for such armour can still be found in Faeo.
However, the ore itself is much harder to find. Only shopkeepers Sadok in the Baurvill Hamlet and Armak in the Settlement of Faytir can get it during "Ore Warehouse opens!" event. Merchants managed to establish business relations with a team of miners who stumbled upon an ancient gnomish vault. Now, shopkeepers get batches of this valuable product from them every now and then.
Sadok and Armak can trade the ore to adventurers for various quest resources. Besides, you will be able to buy more ore from them by paying in gold or diamonds if you prove your allegiance to the legacy of the gnomish people and acquire the Gnome Guardian's Ring or Gnome Treasurer's Ring.