A picture can tell a thousand words...
When you say clip near the rear window is that the plastic one that you put the buckle/ clasp in when its not in use? Alot of the time these fall out of the c-pillar trim due to them usually turning to chalk after 45 years or so. You could try gluing it back or using double sided tape. I wouldn't use them anymore though as chances are they will fail again. If its the clip thats broken you'll have to search one out. To expand your search, TD and possibly later Cortina's use the same type.
With the buckle if you're not worried about originality I'd get some new ones, plenty of suppliers online.
If you do want an original it'll be the same story of searching one out. XC Falcon used the same type but the webbing maybe different.
Another option if you can dig up original parts is to send them to somewhere like this
https://seatbeltsolutions.com.au/services/
They can supply new seatbelts using your original buckles/ hardware.