I have found that the people who do this for a living, will sell you the fabric, but they wont reveal their source. I heard the same thing from one of the river rangers down the Grand. They wanted to sew their own stuff, but had to buy the material retail. And who can blame the people that do it for a living.
I have seen that material a couple of times covering dump truck loads. The next time I see it I think I am going to follow that truck and find out who makes their covers and maybe get the source.
I found something similar at a place that makes awnings but they wanted way too much per yard so I passed.
You had me at 'Hey, Row'