HMS Motorsports carry the harnesses. They bolt into stock seatbelt locations, but should only be considered a
suppliment for light race duty (i.e. AutoX).
http://www.hmsmotorsport.com/store.php?m=12
I would not use them on the street or on the race track, as they will get you killed in a roll-over situation. Stock seatbelts are still the safest in a roll-over unless you've got a full cage.