The vacuum source at the top of a 2002 Chevy Express 1500's 5.0L engine (Vortec 305) is typically the
intake manifold. There are several vacuum ports on the intake manifold. Specific routing depends on what vacuum-operated devices are present (brake booster, HVAC controls, etc.). A diagram in a repair manual specific to your vehicle's year and engine will show the exact location and routing of the vacuum lines.
You won't find a single "vacuum goes here" point. The manifold provides the source vacuum and then branches off to multiple components.
