The stones are only available from GW as part of the Citadel basing kits (there are two, one for Warhammer and one for Warhammer 40,000). Although some complain that these kits are overpriced, I use them myself and find them, if not invaluable, then very helpful when I want to do a cool-looking base very quickly.
But getting pieces of slate needn't be anything like as pricey as that. All you need is a larger piece of slate, a hammer and a hard surface! You can find pieces of slate all over the place in parks and gardens where it is a popular material for controlling weeds in borders. Obviously, I don't recommend picking pieces out of other people's gardens, but if a piece is lying on the pavement, no one will mind if you pick it up! You can also find discarded slate tiles at reclamation yards. A single tile will give you enough slate for basing a whole army. Or check at your local home improvement store (Home Depot in the US, B&Q in the UK). They usually sell slate bits by the 3kg bag-full, but the bags often split and you'll find scattered pieces on the ground nearby. Again, no one will mind if you pick them up from the floor.
I should add that, if you're going to attack a piece of slate with a hammer, the slate will smash in all directions. You should wear thick gloves and safety glasses, and try to surround the slate itself with barriers so that broken pieces don't vanish into the grass.
And if all that sounds too scary, I refer you back to those over-priced tubs from GW.