If you're not committed to keeping things "completely" original and you have the mounting hole along the headliner, you can use original three-point belts from a '72 2002. (At least that's the year mine are from. Other 2002 belts may work as well.)
Only those "in the know" would be aware that they're not correct. They're cheaper than CS belts and more plentiful, and they even have correct BMW markings.
Ian