While searching for the exact same thing you are looking for some years ago, I ran into American Classic Truck Parts inTexas..
www.americanclassics.com.
On their website catalog, they list 67-72 Chev Suburban replacement headliner panels. I have never ordered or purchased from them and this is not a product or company endorsement....just info for you to research. Prices appear very reasonable but again, I have never ordered anything from them. This is, however, the only place I've ever found that advertises replacement headliners for our burbs...If you (or someone else) orders from them, maybe a "Product Review" could follow. Replacement of Burb headliner panels is something many may want/need to do
I recently saw some 4' x 8' sheets of what appeared to be "plastic" panels at the MAN'S STORE (HD or Lowes) that could be used for replacement non-original panels, but (in your case) would be difficult to cut/fit without the originals to use as templates. If I remember correctly a Board Member (BIG ORANGE) may have used these sheets in the fabulous Burb he did some years ago.
Hope this helps,
7T1BURB