Since the APU is soldered to the motherboard, you would have to buy them together. HP has a parts store at parts.hp.com where you can try to find it, but availability and pricing on back stock is usually not ideal. They also have a list of part resellers, which might help. Also, you can lookup HP part numbers for your model notebook at partsurfer.hp.com
You can try used markets, but it can be hard to find the right thing without the HP part number.