I have been told that Plazamate from Dow is the same thing. the only difference is that it has a 1/4" wide by 1/2" deep channel running on the long edges of the foam. It is very small and doesn't seem to get in the way. It is as if they cut the corner off.
Allied building Products has this ($18.25/sheet) and I think Marjam can get it. I bought a piecs and it looks and feels the same as HiLoad 60 (same basic specs too).