Just today I visited my local decorative rocks place, a big yard with bins full of various rocks, about 5 miles away. I picked up 4 rocks ranging from about a foot tall to about 3 inches across. They are very dark gray with white lines and flecks in them. The weight was about 15-20 pounds. The cost was $2. That is the best way to get rocks if you can't go hunting for them out in the wild.
Incidentally I will not be using all of those rocks, so anyone in my area can have the leftovers if they want them. And, the rock place is on Kiefer Road, between Watt and Howe. Very nice place to look for rocks.