As Ryland says, the insulation itself is much more resistant to heat that polystyrene. Polystyrene starts to have trouble around 130F, while polyiso can go above 270F.
I found that around here several lumber yards have it, but did not know it by the name polyiso or polyisocyanurate -- you had to go look at what they actually carried to see.
If its polyiso, it will say polysiocyanurate somewhere on one of the face sheets. You may find that a local place has it, but just does not know it by that name. Atlas and Firestone are two of the common brands.