Buy it from Logan
or if you built the workshop at your house, you can craft it with wood and proper tools
You find it by exploring in the basement tunnels under the tavern, then talk to Blake (blacksmith) about the strange rock. He wants you to get more, so then buy a pickaxe from him so you can mine it underground (it's a great source of money after the initial quest, which is why you're limited to how much you can collect in a day).