How to get Bronze in Hades 2
Where to find Bronze in Hades 2

Bronze is one of the many reagents you'll need to progress through Hades 2. You'll use reagents for things like Incantations and Alchemy at the Crossroads Cauldron, and for unlocking new weapons at the Silver Pool.
Where to find Bronze

There's actually an easy answer to this one: Bronze is found on the surface. The complicated part is opening the path to the surface.
For that, you'll need to progress until you unlock the Shadow Extraction Alchemy (30 Ash, 30 Psyche, and 3 Fate Fabric) to complete the Permeation of Witching-Wards Incantation (3 Moly, 1 Cinder, and 1 Shadow). You'll get both of them during one of your fights with Hecate in Erebus.
What you need Bronze for
Bronze works a lot like the Silver you find in Erebus. You'll need it to unlock new Nocturnal Arms and Gathering Tools at the Silver Pool (the weapons locker) in the Crossroads. You'll also need it for Incantations like the Aspects of Night and Darkness that power up your weapons.