It all depends on whether you want to build sheeted wings or other parts. For wings, I'd go with white Styropor-type of foam, you can find it in almost every hobby shop until 15kg/m3. I found 10kg/m3 (very light) only here: [link=http://www.suterkunststoffe.ch/pdf/produkteuebersicht.pdf]Swiss Composite (PDF)[/link], but haven't tried it yet.
For building hulls or airframes you might want to use blue foam such as Roofmate, again you can find various measures in your local hobby shop.

Regards,
-Fabrizio