Check with International Auto Parts
http://www.international-auto.com/
I found the door seal rubber there for only $3.50/ft. If you send them a sample or picture of the piece, they'll match it for you.
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.