There are literaly dozens of "ruberzied cork" materials. You darn near have to be a materials expert to find what you need. Need to know the actual rubber component, durometer, etc. The other bad thing is most companies that carry such products won't sell in small qty's. It's been a years long project to find a VERY nice gentleman to let me by a special type of fiberglass insulation by letting me tag onto thier orders. Else I was required to by a semi load at a time
I need to buy some "rubberized cork" myself to do some 68 Shelby taillight gaskets. It's not at the top of my priority list at the moment though.
What are you looking to make? How big a piece do you need? what thickness. All I have is about a dozen different 8" x 8" x 1/8" samples.
Might try Ebay. There are people selling some materials on there sometimes.