I'll use just about anything as long as it's 100% plant material. I always start the smoker with newpaper. Like Don, I was using free coffee jute sacks but my source dried up. Be careful getting burlap, it's normally treated with a petroleum of some sort as a preservative. Smell it, if it smells like oil of any form you don't want to use it. Coffee bags are safe as fuel as coffee roasters don't want their beans smelling of petroleum.
Lately I've been using worn out 100% cotton blue jeans (cut into 3" strips) and T-shirts (cut into quarters) as the main smoker fuel. It's easy to wad up and carry a few, it's free, and a wadded strip or two seems to last for hours, even multiple lightings. In a pinch a tight wad of weeds and leaves from around the hive works fine but it doesn't seem to last very long.