Artifact Merchant sells hero artifact fragments. To open this Shop, tap Merchant at the bottom of the Airship page:
Alternatively, you can open the Shop by selecting the desired artifact and pressing Find next to Merchant:
Here, you can spend 400 Artifact Coins to purchase any necessary artifact fragment:
Note: Unlike Artifact Weapons, books and rings are not unique artifacts. For example, Defender's Covenant is not exclusively Astaroth's artifact. You can use it to upgrade Cleaver and many other heroes.