Bagon is found in the Victory Road, a cave located in the north-east of Kanto. It's a rare encounter, but it's not dependent on any blocks or items.
To find Bagon, you'll need to:
1. Complete the game and have access to Kanto.
2. Reach Victory Road in Kanto.
3. Explore the cave and encounter a Bagon.
Keep in mind that Bagon is a rare encounter, so you may have to search for a while before finding one. You can increase your chances by using Repels to avoid weaker encounters.