Where can I get a Lapras in Pokemon White?
Locations To get Lapras: Shaking/Bubbling spots By Surfing at Village Bridge in Unova.
Ways to Obtain Lapras:
(1) Migerate From another game with PokeTransfer for Generation IV.
(2) Trade with another game That has a Lapras.
Sources: http://bulbapedia.bulbagarden.net/wiki/Village_Bridge and http://Pokemondb.net/location/unova-village-bridge