I'm just a lurker, so hopefully someone with actual foamie building experience will come along.
There is a sticky about types of foam (short by foamie standards).
http://tnttt.com/viewtopic.php?f=55&t=48389XPS is what you want. The lack of availability in some parts of the country has been discussed. I think someone suggested calling around to contractors to see if you could buy from them. Don't remember anyone reporting success with this method.
Others have mentioned that the big box stores might order for you, but often only if you buy an entire pallet and pay shipping. I found a few insulation recycling places (Craigslist - materials) that occasionally have the 2" XPS foam. But I'm in the mid-west and all of the big box stores stock multiple styles of XPS.
I think someone used 2 sheets of 1". But, iirc, that was to get a radius. And two 1" sheets will cost more that one 2" sheet.
That's all I can offer in the way of information. But keep asking questions. Foamies are usually friendly.
And they love