You can use the OEM's. OR You can add a Harness bar and a Race Harness(es).
You can NOT modify the seats to use these in SS. You also must retain the OEM seatbelts and reels in the car...allthough you will not use them during the event. However they will work. You MUST use a 6 point harness with stock seats rather than a 5 point.
Most are using either a HardBar or Brey Krause bar with either a Brey Krause or HardBar Lap Belt Kit. These retain the OEM belts and reels.
I also prefer "Clip IN" belts so that you can quickly install the racing harness Lap Belts and Anti Sub straps and remove them on the street.
For the actual belts I would suggest Simpson, Team Tech from HardBar, or Schroth.
There are a couple of write ups and threads in the racing section.
Brey Krause also has a nice Fire Extinguisher mount.
Thanks for the reply DJWorm and a delayed thanks for the PM reply on the brake question. I was curious as to how a 5 or 6 point would connect. I'll head out to the links you gave me and see how this works. I would really like to stay more planted in the seat than the stock seat belts allow.