This company called the foam factory sells different types of foam mats.
Neoprene Sheets, Rubber, Poker Table Foam, Neoprene Pads
According to Sound Deadener Showdown's foam description
Sound Deadener Showdown - Your Source for Sound Deadening Products and Information
It is "Vinyl/Nitrile Closed Cell Foam", well the description on The Foam Factory says that "Technical name is Polyvinyl Chloride Nitrile Butadiene Rubber" so unless I'm missing something, this is the same thing, but instead of paying
$1.10/sq ft, for 1/8" from SDS or
$1.60/sq ft, for 1/8" from SS for Overkill
you get this stuff for $.34 cents/sq ft, for 1/8" aka 32 square feet for 11 bucks lol. They have it in thicknesses up to 2". This seems to be a decent alternative to name brand stuff.
All I really need it for is a decoupler for mass loaded vinyl(which I'm trying to hunt out a cheap supplier for as well).
Let me know what you think. I dont think theres much technology involved here for closed cell foam since its supposed to be an innermediate layer above an anti vibration mat.