If you're looking for a Centauri Carbon hotend, whether you need to replace a clogged or damaged one or you simply want to keep a spare around, replacement hotends are pretty easy to find now.
You can find a replacement Centauri Carbon hotend on Amazon here:
Honestly, I think having an extra hotend around is a pretty good idea if you use your Centauri Carbon a lot. A bad clog, damaged heater, thermistor problem, or other issue around the hotend can take your printer out of commission, and having the entire assembly ready to swap can be a lot easier than trying to repair individual pieces.
Before replacing the whole thing, make sure the hotend is actually the problem. If you're just dealing with filament that won't extrude, I'd first heat the nozzle and try clearing the clog. Also check the extruder gears and filament path because those can cause very similar symptoms.
If the hotend itself is damaged or you have a clog that you simply can't clear, replacing the assembly starts making a lot more sense. The Centauri Carbon hotend combines several of the components responsible for heating and melting your filament, and complete replacement assemblies can save you from messing around with each individual part.
The standard Centauri Carbon uses a 0.4mm nozzle, but there are also different nozzle sizes available depending on what you're trying to print. A smaller nozzle can give you finer detail, while larger nozzles such as 0.6mm or 0.8mm can be useful when you care more about speed and stronger, thicker extrusion than tiny details.
One thing I'd definitely recommend is checking the Amazon listing carefully for compatibility with your exact printer before ordering. The original Centauri Carbon and newer Centauri models don't necessarily use identical complete hotend assemblies, so don't assume something fits just because “Centauri Carbon” appears somewhere in the product title.
For most people, I'd stick with the standard 0.4mm setup unless you have a specific reason to change nozzle sizes. If your original hotend is giving you problems, or you just want an inexpensive spare sitting in your parts drawer, you can check out the Centauri Carbon replacement hotend on Amazon using the link above.