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.
I echo the landscaping/rock supply places. I spent twenty bucks on slate, and had more than enough to supply my 66 gallon malawi tank and my 70 gallon tank. Well worth going through a slightly dirtier place (compared to the clean rocks at a LFS).
Some people just have it made, I cover rock quarry customers in 42 states, and 5 foriegn countries, I can get anything I want, but the security people at the airport look at you real funny when you have 150 pounds of rocks in your carry on bag.:laugh: :laugh: :laugh: