It is not a package, it is distributed as part of the operating system.. it is the default ksh implementation.
The developers have customized it over the years for use on OpenBSD, a few 3rd party projects have ported it elsewhere though.. AFAIK those efforts have stalled.
You can find the source in the CVS tree, or in the src.tar.gz tarballs put out with each release.
http://www.openbsd.org/cgi-bin/cvsweb/src/bin/ksh/