Made some progress! I was able to find out through the partsurfer service that the heat sink's part number is 667268-001. See here for a photo. I'm not sure if I can order it separately, though. I did find some refurbished components through resellers, but no new HP parts yet.
Fortunately, it seems that the same heat sink is indeed used in both low-end and high-end CPU kits, so worst case I can order a cheap CPU kit and put that on a high end CPU.