I did this in my old '70 camper years ago.  The local specialty wood 
supplier had birch plywood in 5' x 5' sheets.

>> <snip> I prefer the plywood panels and trim in my old 71 
>> and am going to reproduce some panels using Baltic Birch or whatever 
>> light vernier plywood I can scrounge.
>>
> <snip>, but one issue is that the headliner panel is slightly wider 
>than 4', perhaps some German standard (?).  I know the hardware stores 
>just carry 4'x8' or smaller; don't know about the speciality lumber 
>stores.  <snip> >I'd be interested in knowing if you find paneling in 
>larger than 4'x8' sheets, though.