http://mars.reefkeepers.net/USHomePa...Denitrator.htm
there ya go.
there really arnt many for sale. Ive only seen two, the 500g one you said and another thats built into a calc reactor.
these are one of the few things I really recommend you just build, they are basicly just an air tight container with a sulphur media and some aragonite to bring the ph back up.
Just make sure you are using a phosphate remover along with it.