you just go south of blackthorn city
in the dark cave near blackthorn city
out of dark cave on route 30 there will come a hiker and he will give you it
if this is the swirl islands . . <-- . . go to this one
go north to dark cave than west to violet city
I don't know how you could have missed it, but it is east of Violet City where route 31 and 30 meet. And yes, the dark, scary opening in the hill is the Dark Cave. There is even a sign if you get confused.
It depends in your playing soul silver or heart gold you catch in one of the dark caves in the water.
You can find it in the Dark Cave or at the Goldenrod Department Store for 2000
just look the walkthrough on youtube
You tell me
Cerulean Cave
the dark cave can be entered on route 45 or route 46
go to icefall cave