Well, you'll need to evolve them from an Eevee, and unfortunately, you can't get an Eevee in Black. If you do get an Eevee, though, here's where you can get the Eeveelutions.
Leafeon: Level up Eevee near Moss Rock in Pinwheel Forest.
Glacion: Level up Eevee near Icy Rock in Twist Mountain
Umbreon: Level up Eevee while full happiness at nighttime.
Espeon: Level up Eevee while full happiness at daytime.
Vaporeon: Use water stone on Eevee
Flareon: Use fire stone on Eevee
Jolteon: Use thunder stone on Eevee
Hope This Helped :D