If you look on gunbrokers they often have old receivers for sale. Find one and buy it or just pm the seller asking him if he will list just the screw you want at an agreed buy it now price. I've bought parts like that there before. Sometimes they have a reciever listed for two years for sale with no interest. They will part it out sometimes if you ask.
You might get lucky on eBay but the screw will not be on an old receiver so you can't be as sure what you are buying. Sometimes you can come across a gun being parted out and know what it fits. I've bought a lot of Remington model 12, Baker, Fox and Lefever parts that way.
