Greg - I have had pretty good success making my own custom decals using GIMP free open source photo editing software and Testors decal paper.
I print them on our regular old HP inkjet printer, let them dry, then seal them with Krylon acrylic satin clear. Just don't apply too much clear, as the decals will not conform to small details as well.
The paper comes in clear or white. If your artwork has any white in it at all you will have to use the white paper, as the inkjet obviously doesn't print white.
Here are a couple of shots of the decals as printed and applied on a locomotive. I put the graffiti on a couple of freight cars.