Since a detailed answer would take much too long, I'm simply going to say this:
A good rule of thumb to find the Card Key is to talk to everyone you've beaten after the battle. One guy in particular (on one of the upper floors, standing next to a table) will say, "Oh, no! I've dropped the Card Key!"
A Poke ball will then appear next to him. Inside is the Card Key.
You get it from the radio director in the basement of Goldenrod. You can obtain it when Team Rocket is invading the Radio Tower.
The card key is required for entering the 12th floor of the Silph company to fight team rocket's leader Giovanni
The card key in Pokemon Heartgold and Soulsilver is located in the Goldenrod Underground.
it is on an island in novella lab and you will find a mailman in blue and he will give it to you
The Card Key can be found in Silph Co on the Fifth Floor.
The card key is located on the 5th floor of Silph Co in Saffron City.
You get the key card from inside the Silph Co. and head to the 5th floor via the elevator.
you only get the card in heartgold and/or soulsilver
Actually its on the fifth floor its somewhere towards the bottom its near a rocket who is near a portal.
you can not
The card key is located on the 5th floor of the Silph Co. building in Saffron City.
The Card Key is found on the floor of the fifth floor of the Silph Co. building in Saffron City.
You have to defeat a certain Team Rocket Agent and he'll drop the Card Key.