Gday Edward,
This is from Eddy Hoek's FJ website. Don't know if it helps. :-/
"
Floorpan tar paper inserts - where do you get them?
So where do you get the 3mm bits of tar board that sit in the reinforcing panels in the floor to level them up? The best way is to get them from an old wreck you see around the place. The second method is to get them from a not so old Holden (but you will have to cut them to shape). The last way is to buy some similiar stuff from a VR commondore. Part No: m40384, 1220mm x 610mm x 2.3mm (about $30)."
quote/unquote.