Give "Aviation Safety Products" a call:
http://www.aircraftseatbelts.com/

They recently rewebbed my four-point Schroth and provided new sliders. I believe they said they have all the parts for a Schroth harness. Maybe they can sell the complete harness? Not sure if they have the rotary buckle.

What about Wings and Wheels? Do the new owners carry Schroth? I bought mine many years ago from Tim at W&W.

Ray