This company makes them at least for the series I
http://www.kaeserfahrzeugbau.ch/auspuffanlagen.htm
I´m sure Roland has them (or knows where to source them) 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.