Landscaping shops and nurseries are about the best place I've found around here, it is still difficult in some areas (like ohio) because in the midwest limestone belt, many landscapers use Flagstone (limestone) Instead of slate. There should be a lanscaping shop around that has some though. Most LFS's carry it, but it's 2-3 dollars a pound and I can get it in the quarries in virginia for $15 a ton or less.