High-quality tin ore is found in What's Mine is Yours. It is obtained after killing a Living rock brawler in one of the mines marked on the Mining sites map. They will automatically be added to the Ore bag when picked up.
Once the player has two high-quality copper ores and two high-quality tin ores they should return to Doric. After speaking with Doric, they should remove the ores from the ore bag and smelt two High-quality bronze bars with them. These can then be smithed into the High-quality bronze sword and High-quality bronze dagger that Doric has asked for.