In the past I've used burlap, sumac, pine needles, paper, and other
things.  I now use the method described in Bee Culture several years
ago.  I built a 1/8" mesh wire cage inside the smoker fitted to the
shape and stuff it with sawdust.  It smoldered overnight recently when I
didn't block the entrance well enough.  I use the smoker for hours
without having to refill the fuel.  It starts with a match and a small
handful of newspaper - soooo easy!.  Gert Walter