Here in the UK we call it underfelt/underlay. This is what your looking for i think
https://www.ebay.co.uk/itm/Car-underfelt-soundproofing-acoustic-felt-sound-deadening-underlay-insulation/182950799448?hash=item2a98b7a858:m:mfiegwhUDkKUgv-nL8MkJQg:rk:9f:0
https://www.ebay.co.uk/itm/Car-underfelt-soundproofing-acoustic-felt-sound-deadening-underlay-insulation/182950799448?hash=item2a98b7a858:m:mfiegwhUDkKUgv-nL8MkJQg:rk:9f:0