Aquarium filter foam.
Laguna brand (made in Italy - no URL) 15 inch square by 2 - 3 inch thick costs 10 UK pounds
Specialist product & not that cheap. But Hey! It will never go mouldy.
seriously, there were all sorts of Shredded wheat type products, Giant loop Velcro pile as well as the foam you are looking for
I would have grabbed some but none of it in red.