You need to find a local manufacture as you will get charged balloon rate shipping and it will slaughter you... Yellow page searches for sculpting, plastic and foam suppliers in your area... Don't be shy about asking around, like maybe a call to the local community college art department...
Short of that house foam and great stuff from your local hardware store... It works wonderful for an on the cheap build up, and with some selective layering you can actually knock out a lot of the initial shape...
Or if you want a DIY solid block knock together a plywood mold and line it with thin painters plastic sheeting and get yourself several gallons of 2 foam, mix pour and watch your creation come to life...