Good luck, I searched high and low. I was even told by a local engineering firm that they could get them for me, and after 4 weeks of waiting, they admitted they could not find them anywhere.
The nearest I found were some "Alan" headed stainless steel ones, but was not sure they would have had the tensile strength needed for the task. Not only that, but by the time I had ordered the 4 I would need for the captive nuts, I was a long way to the cost of buying the complete kit.