If you picked Torchic: Given to you by Steven on Route 120.
If you did not pick Torchic: The Blazikenite can be purchased from the Old Man by the water on Route 114, just outside of Fallarbor Town, for 1500 Poké Dollars. BE careful! He will try to sell you Hard Stones for the same price. This occurs after the Primal Groudon/Kyogre narrative, but can be obtained pre-Elite 4.