Install the OS, including the source distribution. Or, install the OS, and use csup to download the source tree. After that, the sources to everything will be in /usr/src/
Edit
src.conf(5), and add
WITHOUT_DYNAMICROOT. Then rebuild the world. You will now have a statically-compiled base OS (everything under /bin and /sbin).
Alternatively, just copy /rescue/. That's a single crunchgen'd binary hardlinked to a bunch of different names. Which gives you a single 3.5 MB "rescue" disk.