There's somebody on Route 10 who will give you one.
In caves (in the dust) Or in Black City (after the main game is compleated) Hope this helps!
in Lenora in pinwheel forest/twist mountain/dust clouds in caves/black city
you can find dusk stone's in swirling dust in cave's. (random stones/gems and the mole pokemon) and a veteran, south to a hiker, on route 10 will give you a dusk stone.
with action reaplay ds
You can find as many as you can. When you are in a cave (like the Giant Chasm) a swirling dust cloud can contain a Dusk Stone.
i think in the desert resort... :] and if you buy lots of repels go into a cave and run around untill you find swirling dust. go into the swirling dust when it comes up and you can get gems, evolution stones and sometimes pokemon.
black stone
It's called a dusk stone and it evolves murkrow and misdreavus. You can look up more about it on serebii.net.
in pokemon black and white
If you meant dusk stone, you cant. Its not obtainable in gbas,only ds versions
minccino
the light stone isn't in Pokemon white only the dark stone in white and the light stone in black