FastFoot from Fab-Form
Fab-Form seems to have now changed its name to FastFoot but it's the same product.
This membrane sheeting is for constraining wet concrete.
FF50-100 FastFoot Concrete Footing Form Fabric
50" x 100' roll Also 62" and 74" widths
Try buying from Masons in Woodinville.
http://stonetooldepot.com/fast-footformingfabric.aspx $112.57 + $31.05 shipping
The following links no longer work...
62" x 120' roll is about $95.