Where do you get the car key in Pokemon FireRed?

You get the Car Key from Mr. Pokemon, the owner of the Pokemon Fan Club in Vermilion City.

Here's how to get it:

1. Complete the Gym Challenge: You need to defeat the Vermilion City Gym Leader, Lt. Surge, to get the Thunder Badge.

2. Talk to Mr. Pokemon: After you get the Thunder Badge, return to the Pokemon Fan Club and speak to Mr. Pokemon. He will reward you with the Car Key.

The Car Key is essential for accessing Cinnabar Island, where you'll find the Pokemon Mansion and the Seafoam Islands.