http://www.rcfoam.com/
Not sure if your aware of this one (guess you are now) but Rc Foam carries sheets of EPP, Depron, Bluecor in various sizes for the scratchbuilder.
Also a good selection of carbon fiber rod and tube as well as various other RC accessories.
They are located in the U.S. Reasonable shipping rates in U.S. I think they are actual rates but not sure of international rates/shipping. Ordered EPP from them a few times and always shipped out next day. I was very satisfied with their response to questions. Wouldn't hesitate to order from them again.....