To obtain an Inspector's Sacrificial Knife, you need to find and kill Fatui Agents over Level 60+. Upon eliminating these agents, they have a chance to drop Inspector's Sacrificial Knife. Anything above Lvl 60+ also has a chance to drop Hunter's Sacrificial Knife and Agent's Sacrificial Knife.

To get Fatui Agents to become Lvl 60+, you need your World Level to become World Level 5 or higher. The most efficient WL to farm is WL 6, as enemies are between Level 64 and 84. This will give you the most consistent enemy Lvl past 60+. Anything below, you will find them, but not as often.

To get to WL 6, you need to have completed the following.

• Be on World Level 5
• Be Adventure Rank 45
• Clear Ascension Quest 3

They aren't many Fatui Agents in Teyvat, so you'll have to travel a lot to get to them. The fastest way to get around is to use the Waypoints, travel around, kill everything, and move on.

The images below are where you can find Fatui Agents.

In the hands of its lord, this fierce weapon has handled many "debts." No one has eyes in the back of their heads, and this weapon and its related techniques are designed around that weakness.

Alchemy

x3  Agent’s Sacrificial Knife + 50 Mora = x1  Inspector’s Sacrificial Knife