I went to The Foam Source in Boulder for the mattress foam, and have to say it was a good thing I did. Fred Hermsen and his crew know everything about — and can do anything with — foam rubber. With their help I selected a high quality urethane foam (Qualux Q35, see their website for detailed descriptions of this and just about any other type of foam.) They were even happy to make covers for my odd shaped pieces. It is extremely comfortable, and I am very pleased with their quality and workmanship.
Here is the rough sketch that I asked them to fabricate our mattress from.
The Foam Source
2870 28th Street
Boulder, CO 80301