I had a poncho liner lying around and cannot even remember where or when I got it. It may have been at a surplus store, but 15+ years ago.
It is very flat; practically no loft at all. I have been using it with my indoor hammock lined with a SOL heat sheet.
I just ordered two new liners, so I thought I would go ahead and experiment with the "bad" one. It should be enough for inside, and now I can more confidently make ones for outside!