He's looking for REX's predecessor, which was created by the government in March 1929 and placed under the ownership of 86 railroads based on the amount of express that each handled. In 1938 the remainder of the Southern Express company was folded into REX.
I have not seen any evidence of any ARE decals in N and I would doubt that many would exist in any other scale, in as much because people do not know the history of the REX. A quick search of Google turned up nothing, zip, nada... so the only options would be to cut and piece together decal lettering letter by letter, find a suitable font from
Railfonts and print your own (or have someone print them for you) or use dry transfer lettering and make your own decals or lettering...
Clover House offers some suitable letter sets for doing them using dry transfer lettering.
I honestly don't think that folks understand just how the various companies came to be put under REX and because of this the lack of earlier lettering sets of these various companies is understandable. Hopefully more will come to learn about them and how they pertain to the earlier years of railway express with this eventually leading to decals or other lettering being made available. It will, after all, only take time and we all have plenty of that... don't we?