FWIW, Alpha Aviation in U.S. sells replacement Cessna Belts and kits to replace the fixed shoulder straps with inertia reels. I've installed the IR kit in my 172 and prefer them much better than the original fixed straps, which forever dangle haphazardly from the retainers.
FOR OUR INTERNATIONAL CUSTOMERS: All of our belts are stocked with domestic 8130-3 Airworthiness Certificates. If this is acceptable to your aviation authority, we can ship from stock. If a location specific 8130-3 is required, please allow 4 weeks to shipment as we will need to be created at the same time as the belts. EASA; we do not have formal EASA approval. Our belts have been sold & installed all over Europe and the United Kingdom without issue.
About $400/seat plus shipping in US.
Alpha Aviation Inc. - Aircraft Belts - 3 Point Diagonal Shoulder Harness Restraints