If you're still in the wall design phase. Have you looked at buildingscience.com? that's where greenbuilding get their ideas.
they have
lots of techniques to explore. Including a section on high r value walls. Well illustrated and often peer reviewed published reports.
I found the recycled polyiso insulation I used on my "winter bedroom" at a roofing contractor. The sheets came off of failed large building roofs (hospitals, ice rinks etc). Mine were in mint condition.
i searched kijiji. I think in the US you call it craigslist. but a quick scroll through roofing contractors should give you some leads.
My winter bedroom used a modified "perfect wall." I like double stud walls and the larsen truss as well.