the sunroof apparently, when you wrap it around the opening it gets hung up on the sunroof panel. You can get the 1/8 stuff. ask at an upholstery shop and they can probably order some for you. But I have been told by many its really better just to suck it up and get the whole thing from the dealer.
Here is a place that sells it online:
http://www.headlinerexpress.com.
But the freight charges usually mean the Dealer is just as good a price, in my experience. Took 2 days for the dealer to get one in, and it was around $400 plus tax. Its alot for the panel, but it looks great.
Also, you may discover as I did that your middle pillars are also delaminating. They are more than $500 each for some reason, so if that is the case, you will want to spend your time reupholstering those with material. My header board (the part with the sun visors on it) was also delaminating right at the edges, so I glued it up as well.
Good luck.