Here's the easiest way to find them:
- Select Arduino Due from the Tools > Board menu
- File > Examples > SPI > BarometricPressureSensor
- Sketch > Show Sketch Folder
That will open the example sketch subfolder of the SPI library, which is located in the Arduino SAM Boards hardware package. You will find the source code in the various folders of that package. You will also find the toolchain (which contains some source files) used by the Arduino SAM Boards package a few more folder levels up in packages/arduino/tools.