You can catch Lapras every Friday in Union Cave (B2), as Driftloon was to Diamond, Pearl and Platinum.
Downstairs in Union Cave B2, you'll see a large body of water, so Surf south until you can turn east and then north; there's an Elixir on land in the top right corner of the room. After that keep Surfing south until can't keep going down; wash ashore here and fight the male Ace Trainer, who has all three of Kanto Region's Starter Pokemon. On a ledge just above him is a Hyper Potion, so grab that and Surf west to the land just below a female Ace Trainer who uses all four members the Eevee family that debuted in Generation I. Grab the Calcium hidden on the floor behind her, and then head south to snag an Ultra Ball hiding in a jagged rock behind another female Ace Trainer with a Poliwhirl. Surf east from this Trainer and you should find Lapras in the waters. (<- psypokes.com)