Try going to an upholstry shop. They use various thickness and stiffness of foam to do custom interiors. This is what I used. Stuff is cheap. Stuff I got was more of a closed cell polyethelene and will never go bad. Good for sound insulating too.
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.