If you search for 'Mithril' on this site, then go to the 'Object' tab, next to Mithril deposit it will have all the regions that have any Mithril in them, and the locations. Go through these.
that being said I have to agree with the Charred Vale and The Hinterlands. I hardly found any Mithril in the Badlands (maybe 4 or 5 nodes tops and I was there often).