How do you catch to cinnabar island in Pokemon firered?

You cannot catch Cinnabar Island itself. Cinnabar Island is an island location with a Gym and the infamous Pokemon Mansion. Cinnabar Island also has multiple Pokemon you encounter in the wild, including Ninetales, Magmar, Clefairy, and Voltorb. To gain access to Cinnabar Island you will first need to defeat Blaine on Volcano Island. You can access Volcano Island by taking a boat from Vermilion City. Once Blaine has been defeated you can return to Vermilion City and take a boat to Cinnabar Island.