Take a look at the Daltech Force line. They carry both double thick and internal core belts. I recently purchased a Super Bio, although not leather it works perfectly for my Shield using a ISW holster. Also, since it's about 1/8" thick, it solved a problem for me with my phone clip not holding as it did with the double thick belts.
USA Made Gun Belts & Holster Belts – Daltech Force