Simplest thing is to find a rebuilder locally that will rebuild your cores.
Since the pistons don't really ride on the caliper bores themselves, it is safe to assume that the moisture in the system has eaten through the chrome plating and spiked them in place.
I'm pretty sure that Wilkinson and Hall can both rebuild what you have but you might want to get in touch with Stainless Steel brakes in NY.
Paul Fix is the owner, or was, and his wife, Lauren, had a GT5.
If you are going to keep the originals then you definitely want to consider stainless pistons. This is something that they do exclusively and no one else. He also sleeves the bores with stainless.
Google them. They are on the web.
You can't buy the pistons from him seperately.
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.