I'm not as up on it as I used to be but the basics are that the different foam compounds are alloys (mixtures) of various plastic (synthetic) and rubber (natural) chemical compounds that are then made into foam.
I used to (early 90's) get foam from a die cutter that bought the foam from a factory in sourhern Virginia that is no longer in operation. I don't think any of it is no make in the U.S. for environmental reasons.
Maybe one of the smaller tire companies like BSR could do it for you on a custom basis - donuts can be cut by water jet machines that are fairly easy to program.