Natural latex (expensive) or foam with a high density rating and high IFD rating. They higher density, the more resilient; the higher the IFD rating, the firmer.

See link

Here is a link that might be useful: Cushion info.

Thanks so much Tibbrix....the link was most helpful.

Here is another good link with some foam info.
I have bought a couple of foam cushions from them when doing some reupholstery. They are located in Santa Ana, Calif., but also sell online.
I like my cushions to be firm. I bought their Hi-Resilience Q41 for a love seat (2 cushions).

Here is a link that might be useful: Foam Cushion Info

Thanks Laurie!