[Greenbuilding] Insulation fasteners

[bill.allen at verizon.net]

Hi,
I am in the process of putting rigid insulation on an out building.  I am installing 2 layers of 2" styrofoam. I found 3" "nails" (very thin with plastic washers to hold the foam) but am having trouble finding fasteners for the next layer....I'm figuring 6" (2x2"foam+1"sheathing + 1" in the stud).
Any suggestions would be greatly appreciated. Regular nails that big are WAY too expensive.
I will also need a fastener (maybe a screw?) for the next layer...the occasional nailer that will hold the siding. What do people use?
Thanks,
Bill