Hi mitsao,
There’s no way for me to know what it could have been without a more detailed description but it may have been very firm polyfoam (polyfoam and latex have no memory at all). If it had “memory” … then it was probably some type of memory foam.
If it was polyfoam then a local foam shop would probably be a good source or some of the sources in post #4 here would sell it as well if you were comfortable ordering it online.
Some of the better New York options are listed in post #2 here and there is a more categorized list with more detailed descriptions of some of these in post #7 here. Some of these sell mattresses (or toppers) that use firm polyfoam (such as Dixie Beds) if that’s what the material was.
Phoenix