This may not be exactly what your looking for but look for either moon rock or reef rock by Carib Sea. It 's not very shapely but you can get for around $1 per pound, minus shipping.
I second the moon rock or aragocrete. Some rock from lowes may have heavy metals in it, etc. And you can never be too safe. That moon rock can often be found in LFS' for reasonable prices. Sometimes they'll let you bargain too.