Those little Speed Graphic screws are a real nuisance. The original ones tend to rust quite badly, they strip out, they break, they get lost. If a model shop doesn't have them, you can try some of the woodworking shops on-line:
And if not, maybe they can point you in right direction. If you still strike out, let me know - I have a war era 4x5 graphic sitting in my basement that's good for nothing but parts - I might be able to salvage a couple from there.