Unfortunately, they don't appear to reside in Johto. As far as I've seen, the only place they're found is in Mount Moon in Kanto, although...there is a boy that seems very heavily interested in Ghost types and breeding them. I'm sure if you talk to him, he'd find a way to get an egg or two.

[Real pain in the butt, tho.]

Either that or take a trip to Kanto. If you're anywhere near Goldenrod, the train will zip you to and from in no time.

[...And actually that's a plan that's never occurred to her. While her friends are faffing around with badges and such, she could make one of her Secret Trips and go hunting for ghosts.]

You'll be the first to know if I manage to get my hands on one, for what it's worth.