I've always heard great things about harbor aquatics. I've never used their rock, though. IMO the best cheap rock is by checking the classifieds for a while. I got 70 pounds of rock that had been in a reef tank for two years, encrusted with coralline, sponges, foramniferans, feather dusters, all for $50 because a guy had to get rid of it. Check the for sale forum also and sometimes you can find somebody nearby with good rock.