There are some easy DIY instructions out there for making your own seat belts. Buy a used set off of the net and the material from
https://www.gotbelts.com/. I have done this twice, once on the Boxster and once on a 993 turbo. When I did the front seat belts on the 993 I had it cost me $70 for the materials and labor. I had a cobbler do the sewing for me.
There are mixed reviews on the procedure. Below is a link to the DIY I posted on Rennlist in 2010 when I completed it.
Red (& 28 Other Colors) Seat Belts DIY for Under $70 - Rennlist Discussion Forums