Try a local auto upholstery shop or upholstery supplier for a seat belt kit. If there are none in your area, go online to find restoration parts distributors like Trim Parts in Ohio. Search their site for seat belts.
Once you have the seat belt kit, you will find installation instructions inside the kit. You will need to remove the rear seat cushion and then the backrest in order to install the seat belts. Some cars have the nuts already welded to the car, but if not, you will need to drill the holes yourself and reinforce the floor pan with the large washers that come in the kit.
It is a fairly simple procedure and can be done in a couple of hours. Good luck with your project, and enjoy your car for many years.