I'm a student pilot, and all the textbooks and online resources I'm reading show a pitch down as a forward force acting on the top of the propeller (causing the resulting forward force on the right side of the prop, thus a left yaw). Where does this force come from? Isn't a pitch down a downward force on the prop hub, since the rotation is occurring about the airplane's center of gravity (behind the prop)?
The prop is a big gyro. It's the gyroscopic precession force created at the propeller when it's subjected to an input changing its axis of rotation, where, as with any gyro, the inertial resistance to the change in axis acts at 90 degrees rotationally to the input.
The airplane pitches down, rotating the axis (extending forward) of the propeller down, acting through the hub. Being a gyro that is being forced to change its axis vertically, it results in precession feedback acting through the hub at 90 degrees (as if there was a thrusting force at 3 o'clock), yawing the airplane to the left.
It doesn't matter where the prop's axis is in relation to the rotational axis of the airplane in pitch, just that its axis changes as the airplane pitches.
The best way to experience it is to take off in a tail dragger with a large prop, like a Cessna 180, and force the tail up early in the takeoff run before the rudder is having any effect. The swing to the left from propeller precession is pretty strong.
Grab a bicycle wheel, get it spinning, then try to change its plane of rotation.
Rotating objects (such as a frisbee) tend to stay in the same plane of rotation, indeed, this is what makes them valuable as orientation references when visual cues are lost.
A spinning prop is no different in its behavior. When you pitch down, you are changing the plane. One way to see it is to consider the prop as a spinning disk. When you tilt it, the parts of the disk 90 degrees from where you tilt experience the greatest change in direction. This is why pitching at 6 and 12 o'clock produces yaw at 3 and 9 o'clock.
It is the rotational energy of the disk changing direction that causes the yaw.