Hi Ed. I've had a look through my decal book, but no luck I'm afraid. Trying to get letters/numbers in yellow, especially serial numbers, in any scale, is as difficult as trying to find Hitler's dog!
Hand-painting in yellow is a pain also, as you might know, due to the 'thin' pigments in yellow paint. You could try some of the other decal companies, via mail order in US and UK., if you haven't already. Also, Pedinghaus Decals (Germany, I believe) will produce short runs or specials, and they may already have something suitable. Don't restrict to the scale, as a 'smaller' number/letter in a larger scale might do the job. Just Google Pedinghaus, and you'll find them. Another alternative you could try; scan a suitable decal(s) in another colour, eg black, change the font colour, and print-out onto clear decal sheet. Seal with a spray of Johnson's Klear (sorry, 'Future' in the 'States!) and, when totally set, use as normal.
Hope you find what you need, as I know it's a pain and frustrating in these situations.
Terry.