I just got a new RSM powerhead pump from marineandreef.com. They show the RSM black sponge at $6.30, but out of stock. I only found a couple sites selling the black sponge and both I just found were out of stock. I suppose you could call Red Sea and see if they have any in stock. By the way, the sponge now comes with one corner cut off - a Red Sea upgrade so that the sponge doesn't totally plug up and cut the flow in back.
Maybe the type of sponge that some use in a sump could work if big enough to cut to size?