You can purchase it at our store here: Bambu PTFE Tube
Please note that the PTFE tubes are consumables and are at risk of wear, as they may rub against the filament over time. We recommend that you check the PTFE tubes for wear during your maintenance of the machine. If the tubes are worn and not replaced in time, this may lead to feeding failures, filament accumulation inside the AMS, etc.
You can also refer to the Wiki PTFE Tube Wear in AMS and Maintenance Recommendations.
Please watch the video relevant to you in full before replacing the PTFE tube.