Any of the various Pantera vendors carry them. I'm unaware of any cross-over to other cars, but they aren't very expensive. The switch installs with your fingers but you'll need a very thin open-end wrench (Can't remembe the size) to adjust & tighten the lock-nut.
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.