Good luck. That's an old Raleigh proprietary size, and long since out of production. Of course it's possible that some old Raleigh dealer still has some in a dusty drawer, but that's kind of a needle-in-a-haystack search. Your best source is probably another old frame. I changed out the bottom cup and race on one and left the top intact - it improved the front end significantly and was acceptable for the beater campus fixed gear that it was. Changing the fork is an option. Maybe someone will tell you it can be re-threaded to 24tpi; I've never tried that.