Amazon.com - WAHHING 10PCS 2mm thick heating block cotton For 3D printer hotend nozzle heat insulation
Amazon.com - 3d Printer Heating Block - SODIAL(R)10X 2.0mm Thick 3d Printer Cotton Heating Block
Don't forget the Kapton tape if purchasing an aftermarket option.
Amazon.com - Gold Kapton Tape Polyamide High Temp 1/2" (12mm) x 36yds