You might try Brownells on the phone. I looked in their catalog and could not find them. I have bought 4-48 NF screws with slotted heads from them and I could not find them now.
Criterion does not have them? Odd they would make a barrel and not supply the screw needed for the original sight.
I do production machining, if Brownells or Criterion want to order several hundred made from W-1 tool steel or something similar, I might be talked into it.