Geofabric
Dewitt Black 4-Foot by 100-Foot 3oz Weed Barrier Pro Landscape Fabric PBK4100
Available for about $47 for 4'x100' from
http://www.amazon.com/Dewitt-100-Foot-Barrier-Landscape-PBK4100/dp/B0016AJC2U
Brochure: http://www.dewittcompany.com/pdfs/06_landscape_fabric.pdf