http://www.fiberglasssupply.com/
Thats a good source for glassing supplies.
You can find local sources as well. Boat shop, Some paint shops, walmart's autobody section (overpriced).
It is also important to note that polyester resin disolves poly styrene. XPS (extruded poly styrene pink and blue insulation) EPS (styrofoam). If you plan on using these foams you must use epoxy. HD also sels polyurethane foam (cream color) both PE resin and epoxy are safe to use with PU foam.