I would check the landscape supply stores like the others have said. Not free but pretty darn cheap. Also, you get the exact look that you want. They grade everything from type to size. I usually go to Northwest Landscape Supply on Marine Way in Burnaby.
The thing about the landscape supply is that you can get their broken/cast off stuff. At Northwest Landscaping (which is conveniently 2 blocks from my house), I think the broken slate pieces were like $0.05 a pound. I'm going to get a whole load of these for the new tank.