If you find a junk yard headliner, the fabric will probably be sagging and loose in spots. Don't try to glue the fabric back up. It's not worth the time & effort.
On my 96 the previous owner tried to glue the sagging headliner fabric and made a mess out of it. Glue bled through the fabric and there was glue on the trim, grab handles etc. There is no way to glue the fabric back on and make it look good as it has separated from the foam backing material. The foam backing material is what is actually glued to the fiberglass headliner board.
Take the headliner to a pro. The proper way to fix it is to take the entire headliner out of the car (not an issue in this case), CAREFULLY peel off the old material, prep the fiberglass headliner board, and glue on the new headliner material. The fiberglass backing is very fragile and god help you if you break it, crack it chip it, etc.
I found a local upholstery shop that charged me around ~$150. I removed all the interior trim and both of us CAREFULLY removed the headliner out of the car. The color isn't a perfect match but it's close enough. Better than paying VW over $500, if they are still available.
If you do take the headliner out, this is the time to replace the radio antenna base gasket. Good luck finding one of those.