AFAIK, the US Navy air arm were not involved in World War One!
However, US I.D. letters and numbers are available , from WW2 to present, in the major colours and styles, by a number of decal manufacturers, the main one used to be Microscale, now mostly available through Xtradecal, via Hannant's.
There are a few others, and searches of such web-sites as Hannant's and Squadron should find what you need.
If not, broaden the view, and look at other options in lettering decal - civil, different scales etc -- which might find what you need.