The Card Key that is used to open special doors is hidden in the Silph Co building. It is on the fifth floor of the building, near the southern end of the room. However, there is a Rocket agent and a Warp Panel in your way. If you find that you need to heal your Pokemon while in the Silph Co building, keep in mind that a healing station is on the ninth floor - but you need the Card Key to access it.
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You need to head to Level 5, you will see a Pokeball, use the warp panel Back and Forth, the Pokeball will have the Key Card.
go to silph co. its on floor 5 then once you have it go to floor 9
The card key in Pokemon Heartgold and Soulsilver is located in the Goldenrod Underground.
if you got it from the Pokemon mansion it opens the cinabar Pokemon gym
The Card Key is found on the floor of the fifth floor of the Silph Co. building in Saffron City.