I've seen the G-car's patterned headliner, so it's probably factory, but I wouldn't know where to get the material. I assume it's foam-backed cloth like the Stevens cars?
Kenny, the plain "mouse fur" headliner material IS foam-backed cloth, and it's the foam that falls apart after a number of years. I replaced mine several years ago. The '88s had more Mouse Fur than any other Esprit, you guys with later cars are LUCKY that Lotus fitted leather window and door surrounds, and sun visors.
Anyway, for the patterned look, go to a local auto/boat upholsterer and see what they can find. Also check with SJ Sportscars in the UK, Steve might have a source for original style G car material.
The plain 'fur' is a lot easier to find, back when I did mine, fabric stores like JoAnn's and Michael's carried it in tan that was close enough for the later cars where you didn't have to match everything...
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ATTACHED is a pic of my finished job, the most expensive part was the windshield header and sun visors, I had to farm that out professionally. The rest I glued on myself, I can post pics of the process if anyone wants to see them.