You could use fiberglass or cellulose loose fill insulation. Lowes,Home Depot, and Menards will allow you the use of the blower with purchase of any of the above products. Simply drill an approx. 2" hole with a Hole saw attahed to a drill. Set the piece aside to patch the wall.
Insert hose in wall. Try to feed it to the bottom of the cavity. Pull the hose up as the cavity fills. Make sure to keep a rag around the hole, as otherwise the dust will blow all over! After cavity is full use put plug of drywall back in wall to patch. Better yet fun a crown molding to hide the holes.