Try Joann Fabrics also for this. They used to sell it in bags for upholstery work. It is cotton but I don't know how pricey it will be for a project like this. Clothesline would probably be cheaper, but if you want to make a smaller one (even coasters) it would be perfect to try out while you are still looking.
I do upholstery work and I buy mine by the 1250 feet roll so I am all set. I do have the tool to do the braided rugs, but haven't gotten around to that yet.
I might try this out with a rug for my dog. She isn't that big! Hope I can find the time!!
Barb