You'd definitely want "open cell" foam. You can look here at options:
http://www.thefoamfactory.com/opencellfoam/opencellfoam.html
Personally, I bought a sheet of poret and was happy I did.
It looks like their "speaker foam" is the closest to Poret - black, 30 ppi, 2", reticulated. Apparently it is an "ester type polyurethane foam", compared to Poret's polyether, which is supposed to be stronger.
Their price for a 4' x 8' x 2" sheet is $142 (shipping is free), compared to $119 for the 39"x39"x2" Poret. That works out to $4.44 / square foot vs. $10.31 / square foot for Poret.
I wish I knew enough chemistry to be confident that the polyurethane foam would be aquarium safe, because that's a significant price difference.
Having said that, I recently ordered a 13"x13" sheet of Poret and setup a Hamburg Matten filter for a 10g. Super easy to do. It's only been running 2 days, so I can't say much yet, but given other people's positive experiences, I'm fairly confident it will work well.