You can buy them from the Battle Maison for 2 BP each. If you have the money they can also be bought from the pokemart in Laverre City.
As always you can get it then in the Battle Subway/ equivalent, which is the Battle Maison, and you buy them for BP.
You can also buy them for regular money in Laverre City's Poke mart.
Bulbapedia - Vitamin
