Classic parts and maybe another outfit sells a one piece plastic headliner. Not for the thin of wallet though.
https://www.classicparts.com/1947-53...20group%20%231
Depending on one's skill and patience a person might just use a two piece for a pattern and cut a new one piece out of similar upholstery backer board AKA Panel board.
That would probably be a local upholstery supply house thing or see if an upholstery shop would sell you a piece.
Or all jokes aside, watch several episodes of *****in Rides and study how the upholstery dude lays the foam on the inside of roofs of cars to glue in those one piece headliners he does. He does most of that with some sort of foam and an adhesive that works like contact cement.