Ohhh! Great question!

I haven't seen this before, perhaps someplace like Denver fabrics?

FYI, you could probably use a thin wool batting for insulation too.  Wool is a great natural insulator.

ETA:

Have you looked here at Warm Company:  http://www.warmcompany.com/ibpage.html

And at Nancy's Notions: http://www.nancysnotions.com/product/fabric/specialty/insul+bright.do?search=basic&keyword=insulated+fabric&sortby=newArrivals&page=1

I've been looking for the same fabric - what I've found so far is something called Insul-Bright and some thinsulate fabric. I've purchased the thisulate fabric, but hasn't received it yet to see if it will work. It would have been nice if they had included a source for what they used!

Tina