For a black background: I finally managed to get a decent one by going to a company that supplies theatrical fabrics - they have a fabric used for blackout curtains - it has several names, but one of the most common is "commando cloth" (I guess it's used for night missions?). It comes 3m wide, and they happily hemmed a 6m length all round and put a loop pocket across one end so I could slide in the pole of my support frame. Cost? A$177 - which is cheaper than muslin, and much more effective (black muslin tends to let light through from behind - this one is opaque. Underexpose this fabric by half a stop and it goes inky black. It's soft, quite heavy, and pure cotton (treated with flame retardent, so it's safe)
For a white background: the simple answer is overexposure - a stop, or more, and the background will clip to pure white. The white background I used for eight shoots came from Dragon Image - it's a polyester fabric, quite stiff (so it doesn't crease readily), comes 3m wide by any length (under A$20/m), and works fairly well. Sadly, once it does crease, it's pretty much done for - that's what's happened to mine
