There are a couple around, and the sheets I have are Xtradecal (same packaging as what used to be Microscale/Superscale) through Hannant's. (look in Aircraft decals, required scale, RAF Code letters, and you should find them.)
They are available in the various sizes in the three main scales, and, depending on scale, there are normally four of each letter per sheet, with more on the serial number sheets. The latter, of course, can easily be made on the computer and printed onto clear decal sheet as and when required, which also means it's easier to apply them, as they'll be a single decal,rather than individual letters and numbers.