Luciano, thanks for the response.  The Hawk Graphics might have worked, except it's missing one of the shaded letters I need, and I can't be certain that the non-shaded ones are the same height as the numbers I have. 
The GMM sheets actually have the helicopter landing markings I need, but unfortunately it appears that sheet has been discontinued.
I've had great success with Black Lion Decals in the Netherlands making custom decals for me in the past, and given what I'm looking for, I think they'll be the best bet.  Rather than try to put individual letters and numbers together one at a time, they'll just put the whole string together as one decal, and I can have them make as many of them as I need.  
Thanks to everyone, and hopefully before too long I'll start my build log on this project, probably by the end of summer.