A word of caution with the plain white muslin, don't hang it in front of a window, even with it being front illuminated, the window shining through it can create a visable hot spot. Wrinkles in the muslin fabric can be an asset, or on the other hand perhaps a liability.
A lot of folks have used painters drop cloths from hard ware stores with good success. They can be painted and seem to work well.
I personally like the Savage seamless paper grounds. The gray that rbarker mentions is everything he says it is. With lighting alone, you can go from gray to white, subtract the light and back away a bit and it will go to a great black.
I have purchased many remnants from the Belger Company for very little money that work very well for smaller items/products.
Charlie............................