Snivy can be found at the beginning of your adventure, as a starter Pokemon.
As for Buneary, it is only available in Black 2. You can find Buneary in the outside part of the Castelia Sewers. It can be found between 5% or 10% chance you can find it, depending on which grass and what level. Again, it is only in Black 2.
Hope I helped. :)