To stop the fuel supply, you'll need to either:
* Turn the ignition off: This is the primary way to stop fuel flow.
* Disconnect the fuel pump relay: This is a more involved process requiring you to locate the underhood fuse box (usually near the battery) and find the fuel pump relay. Consult your owner's manual for the exact location. Caution: Disconnecting this relay while the engine is running can damage the engine.
* Severe damage to the fuel system: Only a serious accident or catastrophic mechanical failure would physically cut off fuel flow.
Therefore, there's no single answer to "where" because such a switch isn't standard equipment.