They look like old beat up pieces of shipping canvas, what they would wrap cargo pallets with. If you look closely, you can see seems from where they stitched the sheets of canvas together. This is common for large pieces of canvas since no one makes an uniformly super wide and super long piece of canvas. You could try a shipping yard.
For my purposes, which means blocking out light, I bought a 12x18 foot piece of white painters canvas from Home Depot and soaked it in black clothing dye for an hour. The canvas is a dark charcoal, not really black. I never use it as a background, only to black light from spilling into a shot, but my girlfriend who is a food photographer loves it for certain thing. I could see me adding some motor oil stains (or whatever) to it and making it look like what you have posted.