Keder welt. That's what attaches to fabric and slides in the track. Just bought 10' for my rig - from sailrite.com
Another alternative- we used solid pvc awning welt cord. Just slid it into the hem of the tarp and stitched it at the ends to hold it in place. Saved some sewing. We got it from the local awning maker where we got our awning track. They gave us enough to do the awning for nothing. Here is a link to something similar if not the same- comes in different diameters:
http://abbeyextrusio...-pvc-cable-cord
