Hi Mowat

The magic word is "geotextile mat(ting)"... it's indeed not obvious to find, as it actually is more of a product which is commonly used in the construction and gardening industry, - thus with a slightly different initial use. If you are after a natural medium, look for geotextile coco mats. General Hydroponics also offers Coco mats (if I remember well of the size you need) that may be interesting.

Good luck with the project!