Extruded Polystyrene Insulation is a lot more regional that people often seem to assume. Dunno why that is, either why it's available in some areas but not others, or why people are so quick assume it's ubiquitous when it really isn't.
The stuff doesn't exist anywhere within 400 miles of where I live. I'd have to have it shipped to me from out of state, and probably end up paying two or three times its off the shelf price in the process. I don't hold it against people that they have access to materials I don't, but you can only hear/read that "just get some some blue/pink foam, and..." type line so many times before the assumption starts to get irritating.