Where can i get some of this 8mm grey foam from, the stuff that you stick to fibreglass seats.

Seems very expensive but is this what your looking for?
http://cgi.ebay.co.uk/FOAM-POUR-SEAT-SYSTEM-FOR-KITCARS-LOCOST-FURY-STIKER_W0QQitemZ120207141932QQcmdZViewItemQQptZUK_CarsParts_Vehicles_CarParts_SM?h ash=item120207141932&_trksid=p3286.c0.m14&_trkparms=72%3A1301%7C66%3A2%7C65%3A12%7C39%3A1%7C240%3A1318

Camping mat is always popular choice. Cheap too.... And available in a choice of colours....

I think this is what you are looking for;

http://jkcomposites.com/7%27s-page.html

Camping roll mats as said or i have heard on here about using wet suit material?