Check out your yellow pages for local foam suppliers. They can make a template and you can test out the different quality, density, content & combinations available.
Foam Source happens to be close to us...they have a good website
to help you get educated a bit about the options. I would first look close to home so you don't have to pay shipping...although some "include it" in their online pricing.
Both our '64 & '56 had latex foam...which is one of the more expensive & heavier materials. Especially compared to the synthetics that are available today, which weren't available in the 50's - 60's.