I gave up finding the extruded polystyrene. I called Dow, they sent me to a local distributor called Blue Linx. They told me to go to Lowe's or another local lumber company. Both Lowe's and the other place told me I couldn't buy 1-2 sheets, but had to buy 48 minimum. I also called a place called Insulation Supply Company - sounded like a sure thing. The guy I talked to said he didn't sell it and didn't know where to find it.
By that time my patience was running out. I planned to build over this weekend, so I'm using a polyisocyanurate product http://www.rmaxinc.com/Rmatte-3%2004-05.pdf . Picked up at Home Depot for $10 / sheet. I had to glue two 3/4 sheets together, but the R value of each sheet is 5 so should insulate about the same as the 2" polystyrene with R value of 10. I also had some plywood siding lying around that I'm adding for structural strength.
Final gluing of the box today. So far so good.