The kit from Casper's includes the harness and O2. That is all
you need. After that, you can just purchase a heated O2
just about anyplece. As I recall, it uses a Bosch 13474, or
AC Delco AFS 109. You could probably build your own, but,
the Casper's is a clean install, and, by the time you spend the
time effort and energy to build your own, I doubt that you would
save much.