Divinycell foam is a good substitue, and it is available locally in So. Cal. from Aircraft Spruce and Specialty Co.
www.aircraftspruce.com
Divinycell is formable under heat, is carvable, and compatible with epoxy and polyester resins.
The lightest available weight is 3 lbs./sq. ft., and comes in several thicknesses: 3/8", 5/8", 3/4", 1.00", 1 3/4", 2.00", and 3.00". It ain't cheap, but it is good stuff....
Aircraft Spruce offers several other types of foams, and I have used some of them...Polyester, styrofoam, polystyrene, they have it all.
Harley Condra
BVM REP
RAM REP