Any Lumber Yard
Any lumber yard should have fiber glass insulation, if you want to suport a big box store Home Depot should have it unless something freaky is going on, with fiber glass you don't want to install it by packing it, it insulates best when it is loose and fluffy, you can cut, or "tear" it (sort of) but really, cutting it with a sharp utility knife (box cutter style) and a scrap of wood as a cutting board is the best way, craft face (paper glued on to the fiber wool) is normaly used over head, or in areas where you want to staple the insulation in place, otherwise a vapor barrior of some sort of plastic sheating is normaly used between the insulation and the dry wall.
befor insulating make sure to seal up all gaps and cracks that might let in air, water or bugs, I like useing Great Stuff (exspanding foam in a can) but calk works as well, simply sealing up gaps and cracks will help a great deal in keeping a house the tempure you want.
This is the kind of thing I do, super insulation/green building.
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