It's probably a custom heatsink, made for that specific power supply manufacturer.
I'd say get a caliper and start measuring the distances between the holes, the diameter of the holes, where those support pin are relative to the corners...
As a tip, you can also use some warm wax (from candles) to make a small rectangle in which you can press the heatsink just as if the wax was the PCB. Afterwards, you can use the impressions in the wax to determine the distances.