Players can obtain Lapras by talking to the Silph Company employee after beating Gary on the seventh floor of the Silph Company building in Saffron City in Pokemon LeafGreen. One of the main objectives is to get Team Rocket out of Saffron City by saving the president of Silph Company who is on the top floor of the building. Gary will be the second to the last trainer that players will face in the building, with the last being Giovanni.
The following shows how to obtain Lapras in Pokemon LeafGreen version.
After arriving in Saffron City, head straight to the Silph Company building. Because it is impossible to head straight to the top floor to save the president, it is necessary to battle Team Rocket members.
Proceed to the fifth floor, head left and then down up to a warp square. Step on the square to go to the ninth floor. On the ninth floor, step on the warp pad to return to the fifth floor, and head right to obtain the Card Key.
Explore the building, or head straight to the third floor and search for the warp pad that goes to the seventh floor to battle Gary.
After defeating Gary, talk to the Silph Company employee to obtain the Pokemon Lapras.