As has already been said it's really a matter of personal preference, but I like to use rock that won't trap a lot of detritus in it. There is a balance with finding something that is porous and doesn't look like you got it out of the back yard and something with so many crevices that it collects a lot of detritus. Lots of flow can help prevent that, but I have had some rock that would collect a lot of stuff regardless how much flow was on it. As for suppliers, I have always got mine from Premium Aquatics and have been very happy with it.