We bought a foam mattress from Foam Factory www.thefoamfactory.com
They have foam in all thicknesses and qualities. We got the HQ 6" standard double (cost $161), cut it down to size with an electric knife, slit the old cover, peeled it off, pulled it on the new mattress, and stitched it up. Or they will cut a custom and sew a custom cover if you prefer... Great upgrade, and they say the HQ foam lasts an average of 17 years. Better than the 1-2 years for cheap foam. We have been completely satisfied.