There are speciality stores for screws and tools of all kinds. Find the store that caters for the buisnesses in your area and they will find the correct screw, even make it. There are a few places that have these old parts, Helen Howes in the UK will send you one too.
When you open the three pin plug, make a drawing of the wires and where they go; use colors, there's only two cores in each wire and they go in exactly the same way as the old does. The old trick to make the wires stay inside the plug is to make a knot just large enough to keep them from getting pulled through the hole. You can use shrink tubes to neated the wire ends near the plug if needed.