Sara - those are river rocks and they are widely available at garden centers as well as rock/soil dealers. They are frequently used in Koi ponds & stream beds.
I usually boil them in my microwave before adding them to a tank.
yeah, they did real good on the planted tanks, neon tanks, discus tanks, and angel tanks. SW sucks. I just don't understand how they keep no top on the neon tanks. those guys are big jumpers, had like 5 on the floor after a weekend when my mom forgot to close the lid before we went away.