You're right - it's not level specific, but mithril can be crafted after receiving 4th knowledge shard from the Tablet of Tabernum. You can check if you have shard 4 (or any of the shards) in your inventory - they may be back a loooooonnnnngggg way!
Mithril is introduced in the goals about Thurston - this then allows it to drop from items. https://forums.storm8.com/showthread...=1#post1356226
You can also get the mithril dragon (maybe it's one of the items that will be on sale on Black Friday)