I've devised a way to capture normal and displacement maps from real 3D surfaces. It's easy!  Just follow the steps below.

Materials

You need these things:

1. Digital Camera
2. Computer with Photoshop and Nvidia's Texture Tools
3. Movable light-source
4. Subject.  (In this example, Subject is a bowl of peanuts.)

Step 1

With your camera fixed, photograph your subject four times, lighted from each of four directions.  You'll want to experiment with the elevation of your light.  Light from a low angle, but not so low that your subject is obscured by its own shadow.

Step 2

Crop your photographs, and convert them to grayscale.

Step 3

Create a new image (hereafter called "AboveLeft"),  with your Above-lighted photograph for a green channel, and your Left-lighted photograph for a red channel.