Here's what you might be looking for:
* The S.S. Anne Ticket: This is needed to board the S.S. Anne ship, where you can meet the rival, and catch your first Pokémon. You get this from your rival before entering Pallet Town.
* The Silph Scope: This is used to identify disguised people in Silph Co. It is found in the Silph Co. building, but you need to progress through the game and defeat certain trainers before accessing the area where it is located.
* The Secret Key: This key is used to open a locked door in the Pokemon Mansion in Lavender Town. You obtain this key from a trainer inside the mansion.
If you can provide more information about the specific card key you're looking for (e.g., where you heard about it, what it's supposed to do), I may be able to give you a more precise answer.