A reputable source told me that Dennis Quella at Pantera Performance, Colorado has the best ones, but I haven't reordered any yet to verify that (I sent mine back to PPC, Reno in disgust). Pat Mical does some good work as well, but I'm not sure he'll sell just the flares, without doing the conversion.
Apparently most of the other vendors have a single source.
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.