You can't. Youre rival is supposed to give it to you but the game isnt finished yet
Chat with our AI personalities
In Violet City next to the PC in the Pokemon Center
In Pokemon Crystal.
As far as I know you can only get it with Pokemon that already have it. But i expect you could find one in dark places like caves.
Pokemon dark cry version is an illegal game, it is a fake.
The first place you can find Ghastly is in the Sprout Tower located in Violet City.