Hi Chris,
There are any number of companies that can produce decals for you. If you are prepared to order a lot, than Microscale is probably your best bet. While they no longer actively support the O scale market, they will do custom runs. However, I believe the minimum number of sets is 250. So either you need a lot for yourself, or you find takers for the ones you don't need yourself. I would be in for TBOX, FBOX, and GONX sets at a minimum. 10 sets each at a minimum. Just 240 sets each to go!!
One of the advantages of using Microscale is they most likely have the artwork for the sets you seek in HO and/or N scale. The other advantage is the sets they will produce will be silk screened, so producing white decals won't be a problem.
For custom printers that do small orders, you have a variety of options. I would do a search of the site as this topic has come up many times. I have heard of Cedarleaf Custom decals, and definitely Highball Graphics. I know that Highball Graphics has work for Don Smith of Industrial Models. I've only ever heard good things about both companies. I'm sure there are many others out there but those are the two I can think of off the top of my head.
My only beef with Highball is that they state that they reserve the right to add the decals that you have them produce to their product line, as long as they are of real prototype roads. If I produce the artwork for the decals, than I feel like they shouldn't get to use it for free. If they produce the artwork, than no problem. I don't know if this would truly be a deal breaker for me, but it is something I have thought about.
I hope this helps. Do a search here on the forum. I'm sure you will find other examples.
Jim