Fabric.com is a good firm to do business with. Martha Pullen,
http://www.marthapullen.com/ ,
is another great place for everything machine embroidery, heirloom sewing, Dupioni, great laces, etc. Not cheap but if it is used in fine sewing she has it.
When I need Dupioni, I go here
http://www.voguefabricsstore.com/Silk-Dupioni-Fabric/
I started buying from them at the Sewing Expo in Chantilly VA and now order from them online.