ummm....The two most popular foams used in hobby airplanes are:

EPS - Expanded PolyStryrene. This is the foam that drink cups are made from. Usually 0.75 to 1 lbs per cubic foot density is used in models. It can be purhcased in bulk from foam manufacturers (www.houstonfoam.com). It can also be bought in craft stores in sheets and blocks, but the lighter density stuff is less common. This foam is typically used in balsa sheeted wings, turtledecks, etc...This foam is reactive to some glues (CA, Styrene Resins, etc...) and solvents (acetone, gasoline, etc.). Hot wire cutting can be used to produce wings and other shapes.

EPP - Expanded PolyPropylene. This foam is much tougher, rubbery, and heavy and is often seen in electronics and appliance packaging. It can also be purchased from some foam manufacturers, but I have not found an easy source near me.....This foam is also more dense making it less suitable for some aspects of lightweight model construction. In models, it will often be used in these ultra-tough indestructible foamies or in combination with lighter EPS to toughen up certain areas. This foam is fairly inert and not affected by most common glues and solvent used in modelling.

Richard