OpenBSD: Where to seek for the source of programs in the base system?

In OpenBSD, I use pkg_info -E $(which [cmd]) to be supplied with the name of the package/port to the executable/command [cmd]. Having this information I can further look for the source code to the software. However, there are some utilities that seem to be shipped with the base install of OpenBSD, such as the netstat utility. Where would I go about finding the source code of such programs?