Rowaphos is a media that absorbs phosphate, silicates, and arsenic. It is the iron-based version of phosphate removers that has the benefit of not leaching phosphate back into the water when the media becomes exhausted, which the aluminum based phosphate removers do. It has a tendency to get as hard as a brick if placed in a filter bag in a canister filter and has to be kept in a fluidized bed filter like the phosban reactor.
Again, just do a search for Rowaphos on the web and you will find all kinds of places that sell it (maybe from the same place you buy your reactor from).