You can get Lapras from Saffron City, from the top floor of the Silph Co. There is a guy close to the last teleport,what will take you to in front of Giovanni.
Talk to that guy,and he will give you a lvl 25 lapras.
You can also get it from (it's rare) the Icefall Cave in the Floe island.
^^
Hope That Helps
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you need a cheat that can catch a trainer's Pokemon to have lapras. because there is a trainer that has a lapras.
You can find the Lift Key in the Team Rocket Hideout on Pokemon FireRed. It is dropped by a Grunt on basement floor 4 after defeating him.
you find lapras in union cave B2 only on fridays
Union Cave
I don't think you can but if you have an Action Replay you could catch the trainers lapras.