If you will be using the futon as a sofa and a sleeper, consider purchasing an innerspring mattress.
They are less expensive than innerspring mattresses, but serve well for occasional use as a sleeper.
Familiarize yourself with futon, foam, water, innerspring, and air as well as the different manufacturers.
Some mattress fills include cotton, foam and cotton, innerspring, or foam.
The Natural Sleep Store carries both innerspring and natural latex crib mattresses.