what fabric should I buy? Doing this job this winter.
Get a headliner fabric in the color you like. Foam on the back. Stuff is like 55 or 60 inches wide, and you can get enough to do the top and the visors if you need those done. Probably 2.5 yards should do a full length headliner and the visors. T-tops, you obviously need about a yard less. The visors need someone that is fluent in redoing the visors as they'll need to be sewn. Might get away with less, but I don't like ending up not having enough material.
IIRC, the full headliner board in the G-bodies are something around 55" long and 47" wide or something around that. I could be wrong. Been a while since I measured one.
Automotive upholstery suppliers and shops probably can hook you up. Try a local place, as 9 times out of 10 they don't LIKE doing the job, but usually they'll sell you the material and may even give you a good deal on it.
Take a swatch of your dead headliner with you or drive the car over to match up what you need.
Of course, you can always do the internet thing.