Raleigh forks were 26tpi until sometime in the 1980s. Generally the better bikes moved over to 24tpi ISO sizing first. Check before buying a new headset. If you have a 26tpi fork the best course of action is to find a suitable replacement 24tpi or even a 1" threadless fork (basically anything but Raleigh). Raleigh 26tpi headsets are no longer made. I got one NOS from the USA and it was not cheap. You can get away with just the top half, which wears very slowly, and use something else at the bottom; Raleigh headset standards are basically the same as JIS but with the finer thread, and JIS headsets are easy to find (Tange Passage).