Name of fabric insulator found inside motors
This is called sleeving. In this case it looks like uncoated NOMEX, but the specific brand might vary.
This is applied in electromotors and generators where this fiberglass-like sleeving is impregnated with insulating lacquer or resin during dip-sealing or vacuum impregnation.

This material is used because it can handle high temperatures and is not flammable. It also has a much higher kV/mm than regular heat shrink. 

In fact, heat-shrink degrades rather quickly, and can becomes actually conductive, and is typically avoided when long term insulation is required. 
Hence why heat-shrink is more a low voltage repair material.

If you are not going to use lacquer or impregnation I suggest you look for coated fiberglass sleeve. This is coated with polyurethane or acrylic.
A brand name of this could be Vidaflex.