In Pokemon Red Version, you have to go to Celadon City and go to the casino. The casino is located slightly below the Poke Center and a little bit to the left, it is about the size of a gym. Once you have entered the casino go to the back past the counter. There will be a member of team rocket in front of a poster, fight him. After you beat him hit the A button on the picture, it will say something like oh look a switch and then ask you if you want to push it. Say yes, and then proceed to your right, a staircase will be there. This part you just have to fight everyteam rocket member on every floor you can get to. One member will drop the elevator key. It should be a guy in a really secluded spot in the building. Good Luck!! I hope this helps
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You beat a grunt, and when you beat him he drops the LIFT KEY and you can use it to get to the top floor and face Giovanni ;D
beat a team rocket on floor 4 talk to him after you beat him and you get the key
You have to beat the all of the rockets. Press A on one of them and he drops it
Go up the elevator.
If you have completed the Gamecorner area(fought Giovanni and got the Silph Scope), then you'll be fine. The Lift Key's only purpose is so that you may use the Elevator there. Otherwise, you'll have to restart your game.