Google failed me.
I got the Eevee from Bebe, now I want a Jolteon.
There's no exact probability, but Serebii says that it is very rare before the nat. dex and uncommon after the nat. dex.
Just look for this thing.
I can't give you an exact number, but it's really low and aboit the same as all the other stones. You're probably better off just getting one in a trade.