You can mine it in dungeons in Dorgania once you're far enough in the main story to get there. For example the dungeon below Siriveta has some.
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how to get mithril bar?
You can get Mithril Ore in dungeons in Dorgania, and craft it into a Mithril Bar at an alchemy table.
how to get mithril pickaxe?
continue main story yull get have make katrx sword for unbreakable armor fight lace
