dave, you can just make your own, I have in the past. nothing more than aragonite or crushed coral and a little bit of portland cement. not too much. you dont want to fill in all the spaces between the grains of agregate. you can form them in little molds or ice cube trays, or make drop cookies like i did that look a bit more natural. then you have to cure them in a tub of fresh water with vinegar and an airstone for a few months to knock down the high alkalinity of the portland cement. then drop them in your sump to get bio-activated.
GARF has a DIY on how to make em and cure them. they make their own live rock too for all their tanks.