I'm looking into baking sourdough bread; I've read that I must have a stoneware or earthenware baking bowl in order to make it come out with a nice crust. Is this strictly true? Or can I substitute something else, say, baking it directly on my pizza stone with a pan of water underneath to provide steam? 
You can indeed do what you ask - baking on the pizza stone with a steam pan.  It works better to pour hot water directly in the pan when you put the bread in though, rather than just leaving water in the pan.  You get a burst of steam and then a consistent small amount.  It's a method that Peter Reinhart advocates.  The other common option - especially with high hydration doughs - is to bake it in a covered dutch oven for the first half.  This approximates some of the effect of the earthen/stoneware.  The steam from the bread itself keeps the environment moist.  You certainly don't need those pots for fantastic bread.  Tonights loaf - no earthenware ;) (I rushed it for dinner, so the crumb isn't what it could be.)  