Well welcome to our world of specialize screws. This why shops have to keep secure placement of removed fasteners. We don't like over paying for these either. Even as a dealer these are high to get.
Double check the threading as these should SAE screws with SAE hex. IF metric only fits and the 3/8 doesn't then they are likely threaded metric.
First the hex should be a 3/8" as that is what they normally used on these engine with the flange head screws. On top of that these screw heads have a T30 slot which makes easier access to the screws for removal and installation. They also a Red loctite patch so they don't vibrate out.
But I have not found any after market that both the T30 and 3/8" hex that are Flanged. I have to add Blue Loctite to the threads as the Red I have is too strong for standard hand tool removal.
699816
The lowest price per I found is

But if threads are 1/4-20 then you should able it in a flange screw locally, just won't have the T30 slot.