If it is still under warranty (I'm doubtful?) then send it to ASUS as RMA.
If no warranty then you will have to find a heatsink on Ebay. They are not available for purchase separately. Keep in mind that another brand of heatsink may not fit so you would need another Strix 1660 TI heatsink.
A bus station is where a bus stops. A train station is where a train stops. On my desk, I have a work station…