Andrew, check out Joe Shulte's work. He will do custom decals for you or he'll do all the work on relettering a model if that's what you prefer. He has a lot of photos of his work on his homepage at Diecast and Decals and several of them are custom First Gear ambulances and other fire apparatus, but it looks like he will work in any scale. Here's his website:
http://www.diecastanddecals.com/

Location: Woodbridge VA