How do you get the key for cinnabar gym in Pokemon Yellow?

You get the key for the Cinnabar Gym in Pokemon Yellow from the Gym Leader Blaine himself.

Here's how it works:

1. Defeat the Gym's trainers. Work your way through the trainers in the Cinnabar Gym until you reach Blaine.

2. Battle Blaine. Blaine will challenge you to a Pokemon battle.

3. Defeat Blaine. Once you defeat him, he will give you the "Key" as a reward.

With the key, you can then unlock the door to the Gym's main area and proceed to challenge Blaine for the Volcano Badge and the TM for Fire Blast.