This is Owens Corning 703 Rigid Fiberglass Board (RFB) in 24 x 48 x 2 inch pieces.
RFB is probably the most popular material for acoustic absorption. It is typically covered with fabric and mounted on the walls in a studio, control room, or listening room. Although AFB of similar density is equally effective at absorbing sound, RFB is somewhat cleaner to work with and holds it’s shape well.
If you need 6 pieces or more, order in full cases here: Owens Corning 703 2 inch Case of 6