If you're searching for an actual hardened nozzle for the Centauri Carbon, there's a little bit of confusion around this because the nozzle can look like a normal brass nozzle from the outside. The Centauri Carbon actually uses a nozzle design with a brass body and hardened steel tip, giving you the heat transfer benefits of brass while making the part that contacts the filament much more wear resistant.
If you need a replacement or just want to keep a spare around, you can find Centauri Carbon hardened nozzle/hotend options on Amazon here:
A hardened nozzle becomes especially important if you're printing abrasive filaments. Carbon fiber, glass fiber, glow-in-the-dark, and other filled filaments can wear through softer nozzles much faster than regular PLA or PETG.
For most people, I'd stick with a 0.4mm nozzle because it's a good all-around size. If you print a lot of carbon-fiber-filled material, though, you may also want to consider a 0.6mm hardened nozzle. The larger opening can be a little more forgiving with filled materials while also letting you put down thicker lines and potentially print faster.
If your current Centauri Carbon nozzle is printing fine, you don't necessarily need to replace it just because you're planning to try carbon fiber filament. The stock nozzle already incorporates a hardened steel tip. I'd look at replacement if your nozzle is worn, damaged, clogged beyond what you can easily clear, or if you want a different nozzle diameter.
The biggest thing when shopping for one is making sure you're actually buying a Centauri Carbon-compatible nozzle or hotend and not just a generic hardened steel nozzle that happens to look similar. Double-check the compatibility and nozzle size on the listing before ordering.
If you want an easy replacement option, you can check out the Centauri Carbon hardened nozzle/hotend options on Amazon using the link above.