Apparently from what I've read, there are regional differences. It also comes in a variety of thicknesses -- I've seen 3/4-in to 2-in. Also, last time I went to the local "home improvement" store, the only foam sheets I could find has already been scored on 16-in centers. This, I suppose, makes it easier for those who install the stuff on houses and such, but it really weakens the board for us train nuts. I have got around this by buying thinner sheets and laminating them together at 90-degree angles and offset, so that the scored places are supported by intact foam in the adjacent layer.
That said, I am still a BIG believer in the foam. This stuff is great to work with.
Oh - another source: Keep an eye out for houses that are under construction or getting an exterior refit. The insulation crew usually has tons of cut-offs that they are more than happy to be rid of (ask first!). Although it is not good for a base since it is cut up, it is good for building mountains and general filling in.