Evans Wiring is probably a good place to source a lot of misc electrical pieces parts.
I've only worked with the 1974 and the seat belt system may be different as 1974 included a seat belt interlock (one year only). But to answer some your questions:
The wiring for the seats runs under the carpet, if the carpet is still in the car, a previous owner who didn't like the seat belt system may just have stuffed it under there and left it (that's how mine was). It runs from the driver's side over the hump to the passenger side. Under each seat is a circular hole in the carpet that the wiring is supposed to feed out of.
There are 4 connections: 1 goes to the bottom of each seat (weight sensor), the other goes to the bottom of each buckle where it mounts to the trans tunnel. The weight connector is one style, and the seat belt connector is another.
The seat belt buzzer and horn relay are the same (again at least in 1974). It plugs into the top of the interior fuse block. Here is what mine looks like: