couple of months ago I was looking for some weird old screws too. First I tried all the well known hardware stores, but they couldn't help me and said I should ask in an old small & smelly hardwareshop just around the corner, and ofcourse the old guy working there recognized the screws immediately when I showed one to him. He said: These are English screws and haven't been made since the late 70's, but here I have 4 of them. He charged me 1 euro for it.
so try looking in some really old hardware store with employees who have been working there since 1960.